docs(#51): add CONTRIBUTING.md

Add contribution guidelines: branch-from-dev / PR-to-dev, signed commits,
Conventional Commits (with examples), what may/may not change (WebARKit/ vs the
vendored old-ARToolKit5 lib/SRC), the cross-repo PR-pair flow, and build/test
(CMake static lib for WASM/Linux via opencv-em; gtest). README already links here.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: kalwalt

CONTRIBUTING.md

@@ -0,0 +1,67 @@
+# Contributing to WebARKitLib
+
+Thanks for helping improve WebARKit! This is the C/C++ source of the tracker; most
+changes are consumed by the [webarkit-testing](https://github.com/webarkit/webarkit-testing)
+superproject as a submodule.
+
+## Workflow
+
+- **Branch from `dev`** and open your PR **against `dev`** (not `main`).
+- Reference the related issue in the PR description.
+- **Sign your commits** (`git commit -S …`).
+
+## Commit messages — Conventional Commits
+
+All commit messages **must** follow
+[Conventional Commits](https://www.conventionalcommits.org/):
+
+```
+<type>(<optional scope>): <short description>
+```
+
+Common types: `feat`, `fix`, `docs`, `refactor`, `test`, `chore`, `perf`. Examples:
+
+```
+feat(tracker): detect features on a downsampled frame
+fix(#55): populate pose3d so getPoseMatrixCV() isn't zero
+docs: document the pose pipeline
+refactor(#50): remove dead computePose/invertPose
+```
+
+## What you may change
+
+- Active WebAR work lives under **`WebARKit/`** (the OCVT planar tracker).
+- **Do not** modify the vendored **old ARToolKit5** sources under `lib/SRC/**` or
+  `include/AR/**` — those are upstream utilities used by
+  [jsartoolkitNFT](https://github.com/webarkit/jsartoolkitNFT) /
+  [ARnft](https://github.com/webarkit/ARnft) and are kept untouched.
+
+## Cross-repo changes (the PR pair)
+
+A change that affects the WASM build is consumed by webarkit-testing via the
+submodule. Coordinate both repos:
+
+1. Branch from `dev` in **both** WebARKitLib and webarkit-testing.
+2. Make the C++ change here; in webarkit-testing bump the submodule pointer and
+   rebuild (`npm run build-docker` → `build-es6`).
+3. Open a **PR pair** (WebARKitLib → `dev`, webarkit-testing → `dev`); cross-link them.
+4. After the WebARKitLib PR merges, re-point the submodule to the merged `dev` tip.
+
+## Building & tests
+
+- The library is normally built **from webarkit-testing** (WASM) — see its README.
+  `WebARKit/CMakeLists.txt` can also build it as a static lib for **WASM** or **native
+  Linux** (selected via `EMSCRIPTEN_COMP`), pulling OpenCV from
+  [webarkit/opencv-em](https://github.com/webarkit/opencv-em).
+- **Unit tests** (GoogleTest) live in `tests/` and run in CI on every PR:
+
+  ```bash
+  cmake -S tests -B tests/build && cmake --build tests/build && ctest --test-dir tests/build
+  ```
+
+  If you change a config constant or the version, update the matching assertions in
+  `tests/webarkit_test.cc`.
+
+## License
+
+By contributing, you agree that your contributions are licensed under **LGPL-3.0**.