Mention the 'Read As' capability in the 2.16 release notes

Change-Id: I2737279433b69917acf62b0efaa92f7d180265c1
diff --git a/pages/site/releases/2.16.md b/pages/site/releases/2.16.md
index d2cc3ff..f7f7831 100644
--- a/pages/site/releases/2.16.md
+++ b/pages/site/releases/2.16.md
@@ -260,6 +260,13 @@
 On the client side, the same `protocol.version` must be set either in
 the user's `~/.gitconfig` or in the repository's `.git/config` file.
 
+### 'Read As' capability
+
+The new ['Read As' capability](https://gerrit-documentation.storage.googleapis.com/Documentation/2.16.1/access-control.html#capability_readAs)
+allows users to impersonate any user to see which refs they can see.
+
+Note: the capability was added in 2.16 but only documented since 2.16.1.
+
 ### REST API
 
 There are several new REST endpoints and additions to existing endpoints, as described in the [REST API](https://gerrit-documentation.storage.googleapis.com/Documentation/2.16/rest-api.html) documentation.
@@ -506,3 +513,5 @@
 * Fix assignee's status on the change screen.
 
   The assignee field's status showed the change owner's status text.
+
+* Add missing documentation of the 'Read As' capability.