docs: add contribution guide

CONTRIBUTION.md

@@ -0,0 +1,84 @@
+# Contributing to shadowsocks-c
+
+Bug reports, fixes, tests, and documentation improvements are welcome. Please
+follow our [Code of Conduct](CODE_OF_CONDUCT.md) in all project spaces.
+
+## Issues and proposals
+
+Search existing issues and pull requests before opening a new one. For a bug,
+include the version or commit, operating system, build options, steps to reproduce,
+and expected and actual behavior. Include relevant logs and a minimal
+configuration with passwords and other private information removed.
+
+For substantial changes, open an issue first to discuss the problem, proposed
+approach, and compatibility impact.
+
+## Development setup
+
+Fork the repository, clone your fork, and create a descriptive branch from the
+current `master`, such as `fix/dns-timeout` or `feature/new-option`. Do not commit
+directly to `master`.
+
+The default bundled build requires a C11 compiler, CMake 3.20 or newer, and Make
+or Ninja. Pinned dependency sources are included; configuration and compilation
+do not require network access or Git submodules. Install Python 3 for integration
+tests and Bash for shell tests.
+
+Run these commands from the repository root:
+
+```sh
+cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
+cmake --build build --parallel
+ctest --test-dir build -LE memcheck --output-on-failure --no-tests=error
+bash tests/test_ss_setup.sh
+python3 tests/stress_test.py --bin build/bin/ --size 10
+```
+
+Built programs are in `build/bin/`. See the [README](README.md#build-from-source-cmake)
+for build options and [modernization notes](docs/modernization.md) for portability
+and compatibility requirements.
+
+## Style and validation
+
+Match the surrounding code style and keep changes focused. The C formatting
+configuration is [`.uncrustify.cfg`](.uncrustify.cfg); avoid unrelated formatting
+changes and edits to vendored code. Add regression coverage for behavior changes
+and update documentation when commands, configuration, or public behavior change.
+
+Before pushing, run the build and tests above and the CI lint commands below.
+Install `actionlint` 1.7.12 and `ruff` 0.15.6, the versions currently pinned in
+[the test workflow](.github/workflows/tests.yml).
+
+```sh
+actionlint -shellcheck= -pyflakes=
+ruff check --select E9,F63,F7,F82 tests scripts
+git diff --check
+```
+
+Transport and protocol changes should also pass independent TCP/UDP
+interoperability tests. With the shadowsocks-rust `sslocal` and `ssserver`
+executables on `PATH`, run:
+
+```sh
+SS_REQUIRE_INTEROP=1 SS_BIN_DIR=build/bin bash tests/test_interop_rust.sh
+```
+
+The required mode fails when prerequisites are missing; optional local runs may
+skip interoperability tests. Report skips explicitly. CI also covers sanitizers,
+Linux memory checks, static analysis, packaging, and additional platforms; consult
+[the workflows](.github/workflows) for checks relevant to your change.
+
+Preserve existing CLI, configuration, protocol, and public API compatibility
+unless a change has been discussed. For bundled dependency updates, follow
+[the update procedure](third_party/README.md) and preserve upstream licenses and
+notices.
+
+## Pull requests
+
+Open your pull request against `master`. Explain the problem, what changes for
+users, and how you validated the result. Link related issues and describe any
+limitations or platform-specific behavior. Keep unrelated work in separate pull
+requests and exclude generated build output, credentials, and local configuration.
+
+Respond to review feedback and keep the branch current with `master`. Maintainers
+will review the implementation and relevant CI results before merging.

README.md

@@ -15,6 +15,9 @@ Current version: 3.3.6 | [Changelog](debian/changelog)
 
 ## Community
 
+See the [contribution guide](CONTRIBUTION.md) for development setup, testing,
+and pull request guidance.
+
 Everyone participating in this project is expected to follow our
 [Code of Conduct](CODE_OF_CONDUCT.md).