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

tools/clang-format-vs/README → tools/clang-format-vs/README.txt

-This directory contains a VSPackage project to generate a visual studio extension
+This directory contains a VSPackage project to generate a Visual Studio extension
 for clang-format.
 
 Build prerequisites are:
 - Visual Studio 2012 Professional
-- Visual Studio SDK (http://www.microsoft.com/en-us/download/details.aspx?id=30668)
+- Visual Studio SDK (http://www.microsoft.com/en-us/download/details.aspx?id=30668)
\ No newline at end of file