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.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210826 91177308-0d34-0410-b5e6-96231b3b80d8