updated readme to reflect new options

README.md

@@ -8,12 +8,12 @@ Compatible with OpenSeadragon 2.1.0 or greater.
 
 To use, include the `openseadragon-fabricjs-overlay.js` file after `openseadragon.js` on your web page.
 
-To add fabricjs overlay capability to your OpenSeadragon Viewer, call `fabricjsOverlay()` on it. This will return a new object with the following methods:
+To add fabricjs overlay capability to your OpenSeadragon Viewer, call `fabricjsOverlay({scale: scaleWidth})` on it. `scaleWidth` is arbitrary scale for created fabric canvas. 0 corresponds to 0 and `scaleWidth` corresponds to 1 in OpenSeadragon viewport coordinates. This will return a new object with the following methods:
 
 * `fabricCanvas()`: Returns fabricjs canvas that you can add elements to
 * `resize()`: If your viewer changes size, you'll need to resize the fabricjs overlay by calling this method.
 
-See [online demo](http://altert.github.io/OpenseadragonFabricjsOverlay/demo.html) or demo.html for an example of it in use. 
+See [online demo](http://altert.github.io/OpenseadragonFabricjsOverlay/demo.html) or demo.html for an example of it in use.
 
 ## Thanks