Added Changelog entry

Author: Arun Persaud

CHANGES.txt

@@ -9,6 +9,7 @@ Features
 * Give user control over the raw underlying template engine
   (either ``mako.lookup.TemplateLookup`` or ``jinja2.Environment``)
   via an optional ``conf.py`` method ``TEMPLATE_ENGINE_FACTORY``.
+* Better error message in ``nikola serve`` when port is already in use
 
 Bugfixes
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