Tweak documentation.

1. Having "get started", "get involved", and "hacking" makes it hard to find
   how to send patches, so add a link from "get involved" to "hacking".
2. Remove an almost 5 year old note on the test running meachanism changing
   soon.
3. Let "hacking" link to the LLVM developer policy.

llvm-svn: 210826