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
diff --git a/docs/CompilerWriterInfo.rst b/docs/CompilerWriterInfo.rst
index 87add67..681777c 100644
--- a/docs/CompilerWriterInfo.rst
+++ b/docs/CompilerWriterInfo.rst
@@ -107,6 +107,12 @@
 * `Mach-O Runtime Architecture <http://developer.apple.com/documentation/Darwin/RuntimeArchitecture-date.html>`_
 * `Notes on Mach-O ABI <http://www.unsanity.org/archives/000044.php>`_
 
+NVPTX
+=====
+
+* `CUDA Documentation <http://docs.nvidia.com/cuda/index.html>`_ includes the PTX
+  ISA and Driver API documentation
+
 Miscellaneous Resources
 =======================