[Docs] Add CMake Primer document

[Docs] Add CMake Primer document

This document is intended to provide a basic overview of the CMake scripting language for LLVM developers. It was unorthodoxly reviewed for accuracy and content on the CMake developer list:

http://public.kitware.com/pipermail/cmake-developers/2016-April/028300.html

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@268096 91177308-0d34-0410-b5e6-96231b3b80d8