Add CONTRIBUTING instructions

CONTRIBUTING.md

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+# How to Contribute
+
+I love patches!
+
+Because this is a [side
+project](https://opensource.google/documentation/reference/releasing) I released
+while working for Google, you will need to sign a CLA before I can accept them.
+
+## Before you begin
+
+### Sign our Contributor License Agreement
+
+Contributions to this project must be accompanied by a
+[Contributor License Agreement](https://cla.developers.google.com/about) (CLA).
+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.
+
+If you or your current employer have already signed the Google CLA (even if it
+was for a different project), you probably don't need to do it again.
+
+Visit <https://cla.developers.google.com/> to see your current agreements or to
+sign a new one.
+
+### Review our Community Guidelines
+
+This project follows [Google's Open Source Community
+Guidelines](https://opensource.google/conduct/).
+
+## Contribution process
+
+### Code Reviews
+
+All submissions, including submissions by project members, require review. We
+use [GitHub pull requests](https://docs.github.com/articles/about-pull-requests)
+for this purpose.