Add a development guide on how to use git

[postmodern]

Add a guide to /docs/dev/ on basic git usage:

• Cloning your fork of the repo.
• Creating a branch.
  • Mention upcoming version branches (ex X.Y.0 branches).
• Switching back to the previous branch.
• Adding an upstream repo.
• Fetching new commits from upstream.
• Rebasing your branch.