More license headers, add a CONTRIBUTING.md file (also fix README.md formatting) (#81)

CONTRIBUTING.md

@@ -0,0 +1,32 @@
+# How to Contribute
+
+We would 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](https://docs.github.com/articles/about-pull-requests)
+for this purpose.