[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:

IMO, we should implicitly use `multiprocessing.cpu_count()` in `run-clang-tidy` and `clang-tidy-diff` and use that number if user didn't specify `-j` explicitly.

P.S. running lint over all clang-tidy is not so fast now:)
```bash
$ time run-clang-tidy -p build/ clang-tools-extra/clang-tidy/
real    5m27.460s
user    163m43.110s
sys     2m5.180s
```

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