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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@362966 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm