[PATCH] Add user documentation for cpp11-migrate

[Sean Silva]

  Also, you should split this so that the Sphinx infrastructure setup is it's own patch. The content changes can then be reviewed more carefully. Also you will want to ping ddunbar so that he sets up an automated job to build this documentation.

  Alternatively, it might be easiest to just keep this documentation inside of clang/docs/ (it can be linked from clang/docs/ClangTools.rst, which already provides a brief overview of the different tools).

http://llvm-reviews.chandlerc.com/D339