[CONTRIBUTING] Add CONTRIBUTING file

Add CONTRIBUTING file with information about CLA and Google's code of
conduct to follow Google conventions/policies. We should add other
information to this file in the future to make contributing to the
project easier.

.github/CONTRIBUTING.md

@@ -0,0 +1,18 @@
+# Contributing
+
+Thanks for taking the time to contribute to floss! :sparkling_heart:
+
+## Before contributing code
+
+### 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).