Add CONTRIBUTING

CONTRIBUTING

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+# How to Contribute
+
+We'd love to accept your patches and contributions to this project.
+Pull requests are very welcome. Bug reports (via issue) are also appreciated.
+
+There are just a few small guidelines you need to follow.
+
+First, please ensure your pull request targets the `dev` branch; the merge
+automation is unable to import changes to the main branch.
+
+## Contributor License Agreement
+
+Contributions to this project must be accompanied by a Contributor License
+Agreement. You (or your employer) retain the copyright to your contribution;
+this simply gives us permission to use and redistribute your contributions as
+part of the project. Head over to <https://cla.developers.google.com/> to see
+your current agreements on file or to sign a new one.
+
+You generally only need to submit a CLA once, so if you've already submitted one
+(even if it was for a different project), you probably don't need to do it
+again.
+
+Note that the CLA doesn't prohibit the use of coding assistance tools,
+including tool-, AI-, or machine-generated code, as long as these submissions
+abide by the CLA's requirements. In particular, AI tools must not be listed as
+authors or co-authors.
+
+## Code reviews
+
+All submissions, including submissions by project members, require review. We
+use GitHub pull requests for this purpose. Consult
+[GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
+information on using pull requests.
+
+## Community Guidelines
+
+This project follows
+[Google's Open Source Community Guidelines](https://opensource.google.com/conduct/).