docs: improve discoverability of Code of Conduct

- move CoC from CONTRIBUTING to top-level, separate COC file
- add note/link in CONTRIBUTING
- add note/link in README (both at top, and newcomer resources)
- move CoC section in CONTRIBUTING to top of file

PR-URL: https://github.com/nodejs/node/pull/3774


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>