add rotate command to docs

doc/README.md

@@ -159,9 +159,11 @@ Parameters:
  - `repeat_bytes`, Amount of bytes to preload with random data and repeat
     throughout the image slice
 
-## TODO `echo split index delay`
+## `rotate deg bgfill`
 
-Run an echo filter on the given loaded file.
+Rotate the image by `deg` degrees, filling the resulting triangles with `bgfill`.
+
+`bgfill` is a hex string, e.g `#000000`.
 
 ## `quicksave`