Document how to create new commands

[quarnster]

Inspiration can be found in backend/commands. This should also describe how to use sublime.log_commands(True) in Sublime Text to get the exact arguments used to invoke the command in the original editor.

[quarnster]

Obviously there's a big "actually implement missing commands" task here, but I'd prefer to keep a separate issue number for separate commands. Please help open these issues up and to claim one of the commands as yours to implement.

[hkdobrev]

BTW you could use task lists if you want: https://github.com/blog/1375-task-lists-in-gfm-issues-pulls-comments