installation docs: mention optional python-matplotlib-wx package for g.gui.timeline

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@74171 15284696-431f-4ddb-bdfa-cd5b030d7da7

REQUIREMENTS.html

@@ -165,6 +165,10 @@ newer versions are named "python-pillow"
 <a href="http://matplotlib.org/">http://matplotlib.org/</a>
 </li>
 
+<li><b>Python wxPython backend for python-matplotlib</b> ("python-matplotlib-wx", needed for e.g. g.gui.timeline)<br>
+<a href="http://matplotlib.org/">http://matplotlib.org/</a>
+</li>
+
 <li><b>Python requests</b> ("python-requests", needed for g.extension)<br>
 <a href="http://docs.python-requests.org/">http://docs.python-requests.org/</a>
 </li>