README.md: Add link to CONTRIBUTING.md

I think this link also appears when opening pull requests, but one needs to read this document before pushing the branch.

BTW:
- there's also another version of the pull request policy [here](https://github.com/scala/scala/wiki/Pull-Request-Policy) on the wiki, which is more up-to-date. @heathermiller, I guess this needs consolidation?
- @paulp is listed as the only reviewer for "repl, compiler performance", which seems to place on him a possibly unwanted load. I suspect listing a second reviewer is appropriate. (I don't propose removing him unless he asks to be removed).