Add CONTRIBUTING file

This will help to direct those who stumble upon the new github mirror
to the proper channels for contributing code.

BUG=
R=jcgregorio@google.com

Review URL: https://codereview.chromium.org/134663003

git-svn-id: http://skia.googlecode.com/svn/trunk@13044 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed