Create LearnTmux.txt

Quick write-up of using tmux
Update LearnTmux.txt

clean up in prep for markdown
Update LearnTmux.txt
re-order commands

I'm trying to perfect the order so that learning how to use tmux can be reasonably accomplished in one sitting. I feel like I may be leaving out some things that power users would be interested in, but for the sake of someone attempting to find out if they enjoy using tmux I believe it is in decent shape.
Merge pull request #743 from kaernyk/master

[tmux/en] [WIP] Add tmux tutorial