Add section about PolyGerrit in 2.14 release notes

Change-Id: I865a66b0ce5a127d566457b4fc869028617bf7d9
diff --git a/releases/2.14.md b/releases/2.14.md
index a636ecb..c46abcf 100644
--- a/releases/2.14.md
+++ b/releases/2.14.md
@@ -7,6 +7,7 @@
 * Changes can be assigned to specific users
 * HTML emails and new templating framework
 * Secondary index with Elastic Search
+* New [Polymer](https://www.polymer-project.org/) based user interface
 
 ## Important Notes
 
@@ -38,6 +39,18 @@
 
 ### Secondary Index with Elastic Search
 
+### New User Interface ("PolyGerrit")
+
+Gerrit now includes a new user interface, referred to as "PolyGerrit", based on
+[Polymer](https://www.polymer-project.org/).
+
+The UI can be switched between PolyGerrit and GWT by clicking the "New UI" and
+"Old UI" links in the site footer. Alternatively, the UI can be switched by
+adding `?polygerrit=1` or `?polygerrit=0` to the URL.
+
+Note that PolyGerrit is still under development. Most use cases are supported,
+but there are still some missing features compared to the GWT UI.
+
 ## Bugfixes
 
 ## Dependency Updates