Add note about GTK+3 requirement for Pyviz

CHANGES.html

@@ -105,6 +105,7 @@ or with a string value with 'b' (bytes) or 'p' (packets) suffix, such as:
   <li> When using on newer compilers, new warnings may trigger build failures.
 The --disable-werror flag can be passed to Waf at configuration time to turn
 off the Werror behavior.</li>
+  <li> GTK+3 libraries (including PyGObject, GooCanvas2) are needed for the Pyviz visualizer, replacing GTK+2 libraries.</li>
 </ul>
 <h2>Changed behavior:</h2>
 <ul>