docs: add CONTRIBUTING.md

Signed-off-by: Feifan He <feifan@codeweavers.com>

Author: 9Shain

AGENTS.md

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+AI must not be used to generate code for contributions to this project.

CONTRIBUTING.md

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+
+# Contributing to DXMT
+
+## Commit Messages
+
+We use semantic commit messages.
+
+Format: `<type>(<scope>): <subject>`
+
+Valid types:
+- feat: A newly added feature
+- fix: Just a fix
+- refactor: Any changes to the implementation that do not introduce new functionality
+- chore: Any changes to the code that do not change the implementation (e.g. formatting, adding comments)
+- docs: For documentation only (typically .md files)
+- build: For build script only
+- ci: For CI script only
+
+The scope can be any sub-directory in `src/`. If a patch involves multiple scopes, separate them by comma `, `. Scopes can be omitted if there are more than 4, but please try your best to constrain your changes into one single scope. For docs/build/ci, no need to add scope.
+
+The subject is a setence describing the change. Do not capitalize the first letter.
+
+And most importantly, please **sign off** your commit with your legal name.
+
+## AI Policy
+
+We cannot accept contributions made or co-authored by AI/LLM (Large Language Model).
+
+You are still free to use AI to do your own research and share your findings with others (including the developers, but please don't create a PR).