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

[via cfe-commits]

EugeneZelenko wrote:

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

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