Fix dead links in README.md and CONTRIBUTING.md

* CODING.md was renamed to CONTRIBUTING.md a while back.
* Fix link to upstream CONTRIBUTING.md (was pointing at itself,
  follows-up 902f1b7).

Change-Id: I465642b842fb2724a57a5c4d93525a9bf46b39ca
Merge "Fix dead links in README.md and CONTRIBUTING.md"