2013-03-20: added usage information to readme.

added usage information to readme.

diff --git a/Readme.txt b/Readme.txt
--- a/Readme.txt
+++ b/Readme.txt
@@ -22,4 +22,4 @@ becomes
         . j
 
 
-
+Usage: ./wisp.py infile.wisp > outfile.lisp