Adding documentation for clang-tidy.

Summary:
Contains a short user's manual and some instructions on writing
clang-tidy checks.

Reviewers: klimek, djasper

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2880

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@202164 91177308-0d34-0410-b5e6-96231b3b80d8