Add rebase instructions directly in documentation

[mattlangan]

• Shell commands (relative HEAD and explicit sha)
• Fixup/Squash/Drop/Reword

[mattlangan]

Fixed in #23 Published at https://cityofaustin.github.io/ctm-dev-workflow/rebase.html

Still probably needs some improvement but I believe this is at least much more thorough than the rake routes blog post.