Proofread README.git-cl.md.
Proofread README.git-cl.md.

This CL just contains spelling changes, a minor rewording, and addition of backticks for commands.

Review-Url: https://codereview.chromium.org/2296393003

primiano/depot_tools,primiano/depot_tools,primiano/depot_tools,CoherentLabs/depot_tools,CoherentLabs/depot_tools