[llvm][docs] Add step by step git to GettingStarted

Summary:
Due to deleting the git llvm script, folks were asking for better documentation
about how to use git in order to commit to the Github repo. I added some step
by step git commands to make the usage clearer.

Context link: http://lists.llvm.org/pipermail/llvm-dev/2020-May/141640.html

Reviewed By: spatel, mehdi_amini

Differential Revision: https://reviews.llvm.org/D80088