CONTRIBUTING: Add a contribution guide

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+Contributing
+============
+
+License
+-------
+
+`bfs` is licensed under the [Zero-Clause BSD License](https://opensource.org/licenses/0BSD), a maximally permissive license.
+Contributions must use the same license.
+
+
+Building
+--------
+
+`bfs` uses [GNU Make](https://www.gnu.org/software/make/) as its build system.
+A simple invocation of
+
+    $ make
+
+should build `bfs` successfully, with no additional steps necessary.
+As usual with `make`, you can run a [parallel build](https://www.gnu.org/software/make/manual/html_node/Parallel.html) with `-j`.
+For example, to use all your cores, run `make -j$(nproc)`.
+
+### Targets
+
+| Command          | Description                                                   |
+|------------------|---------------------------------------------------------------|
+| `make`           | Builds just the `bfs` binary                                  |
+| `make all`       | Builds everything, including the tests (but doesn't run them) |
+| `make check`     | Builds everything, and runs the tests                         |
+| `make install`   | Installs `bfs` (with man page, shell completions, etc.)       |
+| `make uninstall` | Uninstalls `bfs`                                              |
+
+### Flag-like targets
+
+The build system provides a few shorthand targets for handy configurations:
+
+| Command        | Description                                                 |
+|----------------|-------------------------------------------------------------|
+| `make release` | Build `bfs` with optimizations, LTO, and without assertions |
+| `make asan`    | Enable [AddressSanitizer]                                   |
+| `make lsan`    | Enable [LeakSanitizer]                                      |
+| `make msan`    | Enable [MemorySanitizer]                                    |
+| `make tsan`    | Enable [ThreadSanitizer]                                    |
+| `make ubsan`   | Enable [UndefinedBehaviorSanitizer]                         |
+| `make gcov`    | Enable [code coverage]                                      |
+
+[AddressSanitizer]: https://github.com/google/sanitizers/wiki/AddressSanitizer
+[LeakSanitizer]: https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer#stand-alone-mode
+[MemorySanitizer]: https://github.com/google/sanitizers/wiki/MemorySanitizer
+[ThreadSanitizer]: https://github.com/google/sanitizers/wiki/ThreadSanitizerCppManual
+[UndefinedBehaviorSanitizer]: https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html
+[code coverage]: https://gcc.gnu.org/onlinedocs/gcc/Gcov.html
+
+You can combine multiple flags and other targets (e.g. `make asan ubsan check`), but not all of them will work together.
+
+### Flags
+
+Other flags are controlled with `make` variables and/or environment variables.
+Here are some of the common ones; check the [`Makefile`](/Makefile) for more.
+
+| Flag                             | Description                                 |
+|----------------------------------|---------------------------------------------|
+| `CC`                             | The C compiler to use, e.g. `make CC=clang` |
+| `CFLAGS`<br>`EXTRA_CFLAGS`       | Override/add to the default compiler flags  |
+| `LDFLAGS`<br>`EXTRA_LDFLAGS`     | Override/add to the linker flags            |
+| `WITH_ACL`<br>`WITH_ATTR`<br>... | Enable/disable optional dependencies        |
+| `TEST_FLAGS`                     | `tests.sh` flags for `make check`           |
+| `DESTDIR`                        | The root directory for `make install`       |
+| `PREFIX`                         | The installation prefix (default: `/usr`)   |
+| `MANDIR`                         | The man page installation directory         |
+
+### Dependency tracking
+
+The build system automatically tracks header dependencies with the `-M` family of compiler options (see `DEPFLAGS` in the `Makefile`).
+So if you edit a header file, `make` will rebuild the necessary object files ensuring they don't go out of sync.
+
+We go one step further than most build systems by tracking the flags that were used for the previous compilation.
+That means you can change configurations without having to `make clean`.
+For example,
+
+    $ make
+    $ make release
+
+will build the project in debug mode and then rebuild it in release mode.
+
+A side effect of this may be surprising: `make check` by itself will rebuild the project in the default configuration.
+To test a different configuration, you'll have to repeat it (e.g. `make release check`).
+
+
+Testing
+-------
+
+`bfs` comes with an extensive testsuite which can be run with
+
+    $ make check
+
+Most of the testsuite is implemented in the file [`tests.sh`](/tests.sh).
+This script contains hundreds of separate test cases.
+Most of them are *snapshot tests* which compare `bfs`'s output to a known-good copy saved under [`tests`](/tests).
+
+You can pass the name of a particular test case (or a few) to run just those tests.
+For example:
+
+    $ ./tests.sh test_basic
+
+If you need to update the reference snapshot, pass `--update`.
+It can be handy to generate the snapshot with a different `find` implementation to ensure the output is correct, for example:
+
+    $ ./tests.sh test_basic --bfs=find --update
+
+But keep in mind, other `find` implementations may not be correct.
+To my knowledge, no other implementation passes even the POSIX-compatible subset of the tests:
+
+    $ ./tests.sh --bfs=find --posix
+    ...
+    tests passed: 89
+    tests failed: 5
+
+Run
+
+    $ ./tests.sh --help
+
+for more details.
+
+### Validation
+
+A more thorough testsuite is run by the [CI](https://github.com/tavianator/bfs/actions) and to validate releases.
+It builds `bfs` in multiple configurations to test for latent bugs, memory leaks, 32-bit compatibility, etc.
+You can run it yourself with
+
+    $ make distcheck
+
+Some of these tests require `sudo`, and will prompt for your password if necessary.
+
+
+Hacking
+-------
+
+`bfs` is written in [C](https://en.wikipedia.org/wiki/C_(programming_language)), specifically [C99](https://en.wikipedia.org/wiki/C99).
+You can get a feel for the coding style by skimming the source code.
+[`main.c`](/main.c) contains an overview of the rest of source files.
+A quick summary:
+
+- Tabs for indentation, spaces for alignment.
+- Most types and functions should be namespaced with `bfs_`.
+  Exceptions are made for things that could be generally useful outside of `bfs`.
+- Error handling follows the C standard library conventions: return a nonzero `int` or a `NULL` pointer, with the error code in `errno`.
+  All failure cases should be handled, including `malloc()` failures.
+- `goto` is not harmful for cleaning up in error paths.
+
+### Adding tests
+
+Both new features and bug fixes should have associated tests.
+To add a test, create a new function in `tests.sh` called `test_<something>`.
+Snapshot tests use the `bfs_diff` function to automatically compare the generated and expected outputs.
+For example,
+
+```bash
+function test_something() {
+    bfs_diff basic -name something
+}
+```
+
+`basic` is one of the directory trees generated for test cases; others include `links`, `loops`, `deep`, and `rainbow`.
+
+Run `./tests.sh test_something --update` to generate the reference snapshot (and don't forget to `git add` it).
+Finally, add the test case to one of the arrays `posix_tests`, `bsd_tests`, `gnu_tests`, or `bfs_tests` depending on which `find` implementations it should be compatible with.