Update the readme to reflect subcommand usage

Author: phrohdoh

README.md

@@ -8,6 +8,13 @@ MIT
 
 ## Usage
 
+`jasc_and_gpl` operates in one of two modes:
+
+* `jasc-to-gpl` which converts PAL to GPL
+* `gpl-to-jasc` which converts GPL to PAL
+
 ```sh
-$ cargo run -- --jasc-path /path/to/input.pal --gpl-path /path/to/output.gpl
+$ cargo run -- jasc-to-gpl --jasc-path /path/to/input.pal --gpl-path /path/to/output.gpl
+
+$ cargo run -- gpl-to-jasc --gpl-path /path/to/input.gpl --jasc-path /path/to/output.pal
 ```