[docs] Add 'git llvm revert' to getting started guide

Summary: This documents `git llvm revert rNNNNNN` in the getting started guide for broader visibility.

Reviewers: jyknight, mehdi_amini

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63023

llvm-svn: 362966