Update README.md

Added info about the 'stash' status.
Update 'Hacking' section, fixed small typos.
Merge branch 'master' of github.com:drmad/tmux-git

Bring the README.md modified on github to local master
Merge branch 'master' into dev

Bring update README.md to dev
If the info gathering takes more than 1 second, next time a message will be shown on the status bar