[torchgen] Add documentations for custom namespace support (#81362)
Summary: As titled. After #80780 #79733 #78015 are landed, we are officially supporting custom namespaces on operator and kernel level. I'm documenting this new feature in `README.md`.
Test Plan: Nope
Differential Revision: D37802478
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81362
Approved by: https://github.com/ezyang
