Add package-level doc comments

canvas/canvas.go

@@ -1,3 +1,5 @@
+// Canvas defines a standard interface for images that support drawing
+// primitives.
 package canvas
 
 import "image"

event/event.go

@@ -1,3 +1,5 @@
+// Package event provides a system for broadcasting events to multiple event
+// handlers.
 package event
 
 // A cookie is returned when you add an event handler so you can remove it