update README L.Control extend

README.md

 
 ### L.Control.sideBySide(_leftLayer[s]_, _rightLayer[s]_)
 
-Creates a new Leaflet Control for comparing two layers or collections of layers. It does not add the layers to the map - you need to do that manually.
+Creates a new Leaflet Control for comparing two layers or collections of layers. It does not add the layers to the map - you need to do that manually. Extends `L.Control` but `setPosition()` and `getPosition` are `noop` because the position is always the same - it does not make sense for this control to be in the corner like other Leaflet controls.
 
 ### Parameters