Add initial documentation about the new tools repository and the

Add initial documentation about the new tools repository and the
structure of how we're building concrete tools as well as tooling
infrastructure as part of the Clang project.

This documentation is definitely still rough. If anyone can improve it,
flesh it out, or help structure it in a more natural way, please, help!
=] This is not my forte, and patches here are more than welcome!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161855 91177308-0d34-0410-b5e6-96231b3b80d8