Describe new features in new_element in changelog.

pkg/polymer/CHANGELOG.md

 # changelog
 
 This file contains highlights of what changes on each version of the polymer
 package. We will also note important changes to the polyfill packages (observe,
 web_components, and template_binding) if they impact polymer.
 
 #### Pub version 0.12.2-dev
   * Backward compatible change to prepare for upcoming change of the user agent
     in Dartium.
+  * `pub run polymer:new_element` now supports specifying a base class.
+    **Note**: only native DOM types and custom elements written in Dart can be
+    extended. Elements adapted from Javascript (like core- and paper- elements)
+    cannot be extended.
 
 #### Pub version 0.12.1
   * **New**: When running in pub-serve, any warnings and errors detected by the
     polymer transformers will be displayed in the lower-right corner of your
     entrypoint page. You can opt-out by adding this option to your pubspec:
 
         transformers:
         - polymer:
             ...
             inject_build_logs_in_output: false
@@ skipping 157 matching lines @@
 #### Pub version 0.9.2+3
   * fix [15574](https://code.google.com/p/dart/issues/detail?id=15574),
     event bindings in dart2js, by working around issue
     [15573](https://code.google.com/p/dart/issues/detail?id=15573)
 
 #### Pub version 0.9.2+2
   * fix enteredView in dart2js, by using custom_element >= 0.9.1+1
 
 [6ad2d61]:https://github.com/Polymer/polymer-dev/commit/6a3e1b0e2a0bbe546f6896b3f4f064950d7aee8f
 [3b690ad]:https://github.com/Polymer/polymer-dev/commit/3b690ad0d995a7ea339ed601075de2f84d92bafd