[clang-tools-extra] [clang-tidy][NFC] Add mention of running 'clang-tidy' on changes in Contributing.rst (PR #148547)

[Baranov Victor via cfe-commits]

vbvictor wrote:

> Clang-Tidy pull requests are usually small, so is `-j` really necessary?

Do you have anything against `-j` anymore? Otherwise, I'd merge this PR

https://github.com/llvm/llvm-project/pull/148547