add usage

README.md

@@ -6,6 +6,10 @@
 - ncurses development libraries
 ## Building
 To build `todd` all you need to do is run `make`.
+## Usage
+```sh
+./todd <file.todd>
+```
 ## 3rd Party
 Todd uses the following dependencies:
 - stb_ds - https://github.com/nothings/stb