Improve explanation of geometry flags

Mention in basic flags section that geometry options can accept
percentages and negative numbers as arguments.

README.org

@@ -46,7 +46,7 @@ alt + s
 The positional argument should be the name of a program in =$PATH= (as opposed to the full path to a program) and should not contain any flags (see =-f= if you want to pass additional flags to the program when running it). It should always be the last argument.
 
 ** Basic Flags
-=-w= / =--width=, =-h= / =--height=, =-x= / =--xoff=, and =-y= / =--yoff= can be used to set the window size/position.
+=-w= / =--width=, =-h= / =--height=, =-x= / =--xoff=, and =-y= / =--yoff= can be used to set the window size/position. The argument to any of these can be a number (e.g. =-w 800=) or a percentage (e.g. =-w 100%=). Negative numbers correspond to that many pixels less than 100% of the screen width (e.g. =-w -4=).
 
 =-s= / =--session= should only be used for supported terminals and if the user wants to start/attach to a tmux or tmuxinator session.