git cherry-pick -x bd01ae227bc567fd7953e446236364fc4d110a48
from master into v3-6-test and v3-5-test.
To include our new corporate copyright policy.
Assigning to Karolin for inclusion - this is simply an addition of README.contributing at the top level so doesn't need engineering review.
Pushed to v3-5-test and v3-6-test.
Closing out bug report.