Added the how to contribute page.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402799

docs/docs/main_menu.xml

@@ -72,6 +72,10 @@
             <name>License</name>
             <link>license.html</link>
          </item>
+         <item>
+            <name>How to contribute</name>
+            <link>howto_contribute.html</link>
+         </item>
          <item>
             <name>Index</name>
             <link>term_index.html</link>