[docs] Prominently note that modules are expemental.

And ask for people to try it out and send us bug reports!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177551 91177308-0d34-0410-b5e6-96231b3b80d8
[docs] Point inquisitive users to existing module.map files.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177552 91177308-0d34-0410-b5e6-96231b3b80d8