add link to 2.6 doxymentation to master version

doc/index.dox

@@ -30,6 +30,7 @@ Earlier versions of the documentation are also available on the QGIS
 website:
 
 <ul>
+<li><a href="http://qgis.org/api/2.6">2.6</a>
 <li><a href="http://qgis.org/api/2.4">2.4</a>
 <li><a href="http://qgis.org/api/2.2">2.2</a>
 <li><a href="http://qgis.org/api/2.0">2.0</a>