fix(changelog): add RxJS imports breaking change

Closes #5678

CHANGELOG.md

@@ -46,7 +46,16 @@ import * as core from 'angular2/core';
 
 * Http's MockBackend is no longer exported from the `angular2/http` module. It's now available at `angular2/http/testing`
 
+* Operators and Observables from RxJS (e.g. .map(), .toArray(), .toPromise(), etc ) now need to be explicitly imported (once per operator in your app)
+  ```
+  import {Observable} from 'angular2/angular2/'
+  import 'rxjs/operators/map';
+  import 'rxjs/observable/interval'
   
+  Observable.interval(1000).subscribe(...);
+  
+  http.get('foos.json').map(res => res.json()).subscribe(...);
+  ```
   
 <a name="2.0.0-alpha.47"></a>
 # 2.0.0-alpha.47 (2015-12-01)