7zip filter: minimum docs and test

website/features.html

@@ -229,6 +229,14 @@ application/x-tar = execm rcltar
         module and the <a
         href="http://www.rarlab.com/rar_add.htm">unrar</a> utility.</li>
 
+        <li><span class="application">7zip</span> archives (needs 
+          <span class="command">Python</span> and  
+          the <a href="https://pypi.python.org/pypi/pylzma">pylzma
+            module</a>). This is a recent addition, and you need to
+            download the filter from
+          the <a href="filters/filters.html">filters pages</a> for
+          all Recoll versions prior to 1.21.</li>
+
         <li><span class="application">iCalendar</span>(.ics) files
         (needs <span class="command">Python, <a href=
         "http://pypi.python.org/pypi/icalendar/2.1">icalendar</a></span>).</li>