Add txt file extension to VS solution's README

This matches other README.txt files in LLVM and makes things more obvious on
Windows where it's likely to be read. CRLFs are retained for the same reason.

Also fix Visual Studio product name.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195420 91177308-0d34-0410-b5e6-96231b3b80d8