[Docs] Add information on how to contribute to the README

[Docs] Add information on how to contribute to the README

I noticed after reading a few GitHub comments that the information
about contributing to V8 is hard to find. I think it makes sense
to add this information to the README.

NOTRY=true
R=jochen@chromium.org, seththompson@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1342803004

Cr-Commit-Position: refs/heads/master@{#30730}