update readme

README.md

@@ -1,2 +1,5 @@
 run `make` to install it, then do `make DIR=/wherever/your/files/are/ run` to
-run it
+run it. All of the converted files will be placed in a directory called
+`shortened` under the path you supply.
+Requires either an OSX or Linux environment, plus the program `ffmpeg` to be
+installed