Add example to Canvas documentation

doc/canvas.md

@@ -2,7 +2,6 @@ Tabris.js Canvas
 ================
 
 Canvas is a composite which allows the user to draw on it using a canvas context. Canvas context provides a drawing API. It is a subset of the HTML5 [CanvasRenderingContext2D](https://developer.mozilla.org/en/docs/Web/API/CanvasRenderingContext2D).
-.
 
 To create the canvas context, invoke the following method:
 
@@ -14,7 +13,17 @@ tabris.getContext(canvas, width, height);
 * width - the width of the drawable area
 * height - the height of the drawable area
 
-*Pre-Release Note: This method will likely be moved to the canvas itself.*
+Example:
+
+```javascript
+tabris.create("Canvas", {
+  layoutData: {left: 0, top: 0, right: 0, bottom: 0}
+}).on("change:bounds", function(canvas, bounds) {
+  var ctx = tabris.getContext(canvas, bounds.width, bounds.height);
+  ctx.moveTo(0, 0);
+  // ... 
+}).appendTo(page);
+```
 
 The following CanvasRenderingContext2D methods are supported: