add that firebase-collection is now firebase-query

Author: Taylor Savage

firebase-collection.html

@@ -15,6 +15,8 @@
 **Note: This element is for the older Firebase 2 API**
 For the latest official Firebase 3.0-compatible component from the Firebase team,
 see the [polymerfire](https://github.com/firebase/polymerfire) component.
+In this new component, the `firebase-collection` element has been replaced
+with [`firebase-query`](https://github.com/firebase/polymerfire#firebase-query)
 
 An element wrapper for the Firebase API that provides a view into the provided
 Firebase location as an ordered collection.