Add CONTRIBUTING info

Author: bramus

CONTRIBUTING

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+# How to contribute
+
+We'd love to accept your patches and contributions to this project.
+
+## 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 for this purpose. Consult
+[GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
+information on using pull requests.