Add a note about floating windows to README.
Add a note about floating windows to README.

olsonpm/Amethyst,egabancho/Amethyst,ianyh/Amethyst,ianyh/Amethyst,cevaris/Amethyst,ianyh/Amethyst,ianyh/Amethyst,m4rw3r/Amethyst,cevaris/Amethyst,egabancho/Amethyst,m4rw3r/Amethyst,olsonpm/Amethyst,ianyh/Amethyst,hooplaaa/Amethyst,hooplaaa/Amethyst