Mention mandatory tab

README.md

@@ -11,7 +11,7 @@ Comment lines in the file start with `#` which are ignored, along with empty lin
 
 Each animation starts with:
 `animation MyAnimationName`
-Followed by a collection of definition lines:
+Followed by a collection of definition lines, which always have a leading single tab:
 - `frame-origin x y`
   - Where `x` and `y` are unsigned integers.
   - Where is the first sprite frame located in the spritesheet.