[clang-tidy] Update documentation

Summary: We have 2 scripts add_new_check.py and rename_check.py to bootstrap a new check creation. There is also clang-query to aid with the matcher creation. These were not mentioned in the current document, add them so it's available to the masses.

Reviewers: alexfh

Patch by Deniz Türkoglu!

Differential Revision: http://reviews.llvm.org/D16944

llvm-svn: 260098