[clang-tidy] add options documentation to readability-identifier-naming checker

Summary:
The documentation for this clang-checker did not explain what the options are. But this checkers only works with at least some options defined.

To discover the options, you have to read the source code. This shouldn't be necessary for users who just have access to the clang-tidy binary.

This revision, explains the options and gives an example.

Patch by MyDeveloperDay.

Reviewers: JonasToth, Eugene.Zelenko

Reviewed By: JonasToth

Subscribers: xazax.hun, Eugene.Zelenko

Differential Revision: https://reviews.llvm.org/D56563

llvm-svn: 351261