[clang-tools-extra] [clang-tidy][NFC] add '.clang-tidy' config for clang-tidy project (PR #147793)

[Carlos Galvez via cfe-commits]

carlosgalvezp wrote:

Thank you for driving this, LGTM!

One last thing I'm missing: can you update the clang-tidy contributors guide, to explain that we expect people to run clang-tidy as well (and what command to run)? Since it's not currently enforced in CI.


https://github.com/llvm/llvm-project/pull/147793