Update badges in README
Update badges in README

issuestats.com seems to have moved on, using shields.io instead.
i3/i3,Airblader/i3-original,stapelberg/i3,Airblader/i3,stapelberg/i3,stapelberg/i3,i3/i3,Airblader/i3,i3/i3,Airblader/i3,Airblader/i3-original,Airblader/i3-original,Airblader/i3-original,Airblader/i3-original,Airblader/i3,i3/i3,stapelberg/i3