first draft of a written policy around git repos

This is a frist draft of a set of policies around new git repos and how we grant write access to our GitHub organisation.

This proposal is based on the discussions in:
https://github.com/llvm/llvm-iwg/issues/40
https://github.com/llvm/llvm-iwg/issues/51

Differential Revision: https://reviews.llvm.org/D111723