docs: Add CONTRIBUTING.md

Point new contributors at develop for PRs, kernel-style commit
subjects, What/Why/Test PR bodies, and safe issue citations under
Gitea merge-text closing rules.

CONTRIBUTING.md

@@ -0,0 +1,36 @@
+# Contributing
+
+## Pull requests
+
+Prefer pull requests against `develop`. The repository default is `master`
+or `main`; `develop` exists for ongoing integration.
+
+## Commits
+
+Subject form (kernel-style):
+
+    area: Imperative summary
+
+- Imperative mood after the colon (`Add`, `Fix`, `Remove`, …).
+- No trailing period on the subject. Aim for about 75 characters total.
+- Body states the problem and why the change is right.
+- One logical change per commit. Skip the body only when the subject alone
+  is obvious to a reviewer.
+
+## Pull request description
+
+- **What** — what changed
+- **Why** — problem and impact
+- **Test** — concrete steps a reviewer can run
+
+Do not leave the description as “see commits”.
+
+## Issues
+
+Gitea closes issues when a bare issue reference appears in merge text.
+Cite leftover issues by full URL. Only close an issue when the merge
+actually finishes that work.
+
+## License
+
+License: see LICENSE if present; otherwise TBD by owner.