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

src/tests/intro.md

@@ -70,10 +70,12 @@ package tests:
 
 Tidy is a custom tool used for validating source code style and formatting
 conventions, such as rejecting long lines. There is more information in the
-[section on coding conventions](../conventions.md#formatting).
+[section on coding conventions](../conventions.md#formatting) or the [Tidy Readme].
 
 > Examples: `./x test tidy`
 
+[Tidy Readme]: https://github.com/rust-lang/rust/blob/master/src/tools/tidy/Readme.md
+
 
 ### Formatting