[docs][refactor] Add refactoring engine design documentation

This commit adds a refactoring engine design document that talks about the
design and provides several example of how the engine can be used.

Differential Revision: https://reviews.llvm.org/D37976


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@314509 91177308-0d34-0410-b5e6-96231b3b80d8