docs: add some documentation for building on Windows

This covers how to build libc++ for Windows.  This allows others to
replicate the MS ABI style build for libc++.  It only depends on msvcrt
as it uses the Windows threading model and the Windows ABI and can serve
as an ABI compatible replacement for msvcprt.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@294705 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed