update readme and contributing.md (#63843)
Summary:
1. In fact, Visual Studio isn't supported as CMAKE generator
2. I was asked many times why there's error as 'Could NOT find OpenMP'
3. Add Newly added Best Practices link in contributing.md
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63843
Reviewed By: seemethere, heitorschueroff
Differential Revision: D30514095
Pulled By: janeyx99
fbshipit-source-id: 76715a1d8c049122546e5a7778cafe54e4dfd5d6
