update README

README

@@ -1,10 +1,13 @@
 Creates a simple overlay for the current buffer that shows non-staged git chunks.
 
 Lines around additions/deletions are shown blue, additions in green, deletions
-won't show up at all.
+in red.
 
 Just copy it somewhere in your load-path, or add the directory containing the
 git-overlay.el file with (add-to-list 'load-path "/path/to/repo/")
 
 then just (require 'git-overlay) and do M-x git-overlay in a buffer with a file
 from a git repository open.
+
+Calling git-overlay twice will toggle the overlays on and off. If you change
+the buffer while the git overlays are shown the will disappear.