docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to FastFlowLM
+
+## Build
+
+Clone **recursively** — the `third_party/tokenizers-cpp` submodule is required and a
+non-recursive clone will not build.
+
+```bash
+git clone --recursive https://github.com/ROCm/FastFlowLM.git
+cd FastFlowLM/src
+cmake --preset linux-default     # or windows-default / windows-vs18
+cmake --build build
+```
+
+Needs CMake ≥ 3.22, a C++20 compiler, and Ninja (recommended). Linux system packages are
+listed in [docs/linux-getting-started.md](docs/linux-getting-started.md). Always configure
+through a preset — `FLM_VERSION` and `NPU_VERSION` are required and the presets supply them.
+
+**Rebuild fully after pulling.** Commits routinely refresh `.xclbin` kernels and prebuilt
+engine libraries without touching a single source line, so a `git diff` that looks empty can
+still change behavior. Delete `src/build` when in doubt.
+
+## Pull requests
+
+Open a PR against `main`. Use Conventional Commits — `feat:`, `fix:`, `docs:`, `chore:`,
+`ci:`, `refactor:` — with an optional scope like `chore(hrx):`. No sign-off needed.
+
+Rebase before pushing, and match the surrounding code (files use `/// \file`, `/// \brief`
+Doxygen headers). Don't edit vendored code: `src/include/nlohmann/`, `src/include/minja/`,
+`third_party/`.
+
+CI must pass: Windows portable + MSI + HRX, Linux portable + HRX, and a four-distro `.deb`
+matrix.
+
+## Gotchas
+
+**Adding a model?** Both NPU backends need its engine library. The link list in
+`CMakeLists.txt` is unconditional, so an engine that lands only in `src/lib/xrt/` breaks the
+`FLM_USE_HRX=ON` build at link time while the default build stays green. Also register it in
+`src/model_list.json` and add `src/xclbins/<MODEL>-NPU2/`.
+
+**Adding a docs page?** It's invisible until you add it to the hardcoded `docs_nav` list in
+`docs/_config.yml` — the sidebar is not generated from front matter. Give the URL a trailing
+slash; the active-page highlight is an exact match on `page.url`.
+
+## Bug reports
+
+Include your OS, NPU driver version, `flm version` and `flm validate` output, and the command
+that failed. Most startup failures are driver-version related — the docs recommend a newer
+driver than the binary enforces, so report the version you actually have.
+
+## Licensing
+
+Source code is MIT ([LICENSE_RUNTIME.txt](LICENSE_RUNTIME.txt)). The NPU kernel binaries in
+`src/xclbins/` and `src/lib/` are **not** open source — see [TERMS.md](TERMS.md). They're
+built from a separate toolchain and refreshed by maintainers, so if your change needs new
+kernels, open an issue first; a source-only PR won't be enough.