[PATCH] Qt Creator help/documentation file for Clang

[Konrad Wilhelm Kleine]

  Since the CMake variables `LLVM_ENABLE_DOXYGEN_QT_HELP` and `LLVM_DOXYGEN_QHELPGENERATOR_PATH` are already documented in `$LLVM/docs/CMake.rst`, I think the most important parts are covered.

http://llvm-reviews.chandlerc.com/D2965