Add start of user documentation for NVPTX

Summary: This is the beginning of user documentation for the NVPTX back-end.  I want to ensure I am integrating this properly into the rest of the LLVM documentation.

Differential Revision: http://llvm-reviews.chandlerc.com/D600

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178428 91177308-0d34-0410-b5e6-96231b3b80d8