Rollup merge of #146460 - simp4t7:add-tidy-readme, r=clubby789

Add tidy readme

This PR adds a Readme to `src/tools/tidy`. Basically just explains how `tidy` works and covers all of tidy's checks, directives, and how to use tidy. I tried to add a bit more detail on some of the options like `--extra-checks` and the style directives that aren't really documented well elsewhere.

Planning to link to this in the dev guide.

Closes: rust-lang/rust#129368

Author: Zalathar