[tmux/en] Add tmux tutorial [WIP]

I use tmux quite often and think that it does help simplify development, so why not offer a quick and dirty write up of basic usage? I've added the bare essentials needed to manage multiple tmux sessions/windows in a (hopefully) mindful order so that it's easy to digest, and is similar to a cheat sheet.

I imagine most users will have at least heard of tmux/screen, so I will save the fluff for the man pages and leave only commands with a brief introduction.