Add CONTRIBUTING.md
Add CONTRIBUTING.md

R=nweiz@google.com

Review URL: https://codereview.chromium.org//1085363004

grouma/test,iglherme/Guttiest,grouma/test,apwilson/test,iglherme/Guttiest,kevmoo/test,greglittlefield-wf/test,apwilson/test,d2m/test,d2m/test,kevmoo/test,d2m/test,iglherme/Guttiest,greglittlefield-wf/test,apwilson/test,greglittlefield-wf/test,kevmoo/test,grouma/test