Add info on how geometry formats work.

README.md

@@ -188,7 +188,8 @@ geometry and a format, which the file will be converted to, like so:
 
 This will convert the "thumb" style to a 32x32 square in png format, regardless
 of what was uploaded. If the format is not specified, it is kept the same (i.e.
-jpgs will remain jpgs).
+jpgs will remain jpgs). For more information on the accepted style formats, see
+[http://www.imagemagick.org/script/command-line-processing.php#geometry](here).
 
 Multiple processors can be specified, and they will be invoked in the order
 they are defined in the :processors array. Each successive processor will