Currently, CONTRIBUTING.md is not rendered by g3doc due to being in an unsupported location:

https://g3doc.corp.google.com/third_party/tink/CONTRIBUTING.md?cl=head

This fixes that problem and still works in the github context:

"To help your project contributors do good work, you can add a file with contribution guidelines to your project repository's root, docs, or .github folder."

https://help.github.com/articles/setting-guidelines-for-repository-contributors/

PiperOrigin-RevId: 197563787
GitOrigin-RevId: ac25aa8817f5e8c8cca9dcd2f64ab8891bd98bda