[BE] add readme for .github, located in .github/scripts
Add a README for .github, located in .github/scripts because putting it in .github overwrites the repo README.
Content is copied from https://github.com/pytorch/pytorch/pull/72943/files, with a few modifications.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74993
Approved by: https://github.com/seemethere, https://github.com/janeyx99
