Document new vector API in CHANGES.html

CHANGES.html

@@ -54,6 +54,8 @@ us a note on ns-developers mailing list.</p>
 <h1>Changes from ns-3.26 to ns-3.27</h1>
 <h2>New API:</h2>
 <ul>
+<li>Added <code>Vector{2,3}D.GetLength ()</code>.</li>
+<li>Overloaded <code>operator+</code> and <code>operator-</code> for <code>Vector{2,3}D</code>.</li>
 <li>Added iterator version of WifiHelper::Install() to install Wi-Fi devices on range of nodes.</li>
 <li>Added a new attribute in TcpSocketBase to track the advertised window.</li>
 <li>Included the model of <b>TCP Ledbat</b>.</li>