Adding contributing to VOLTHA page with new core maintainers guidelines

Change-Id: Iccae0095f3f1eb40cb619238443679ecd9c5caee
diff --git a/index.rst b/index.rst
index 5908500..8c51b20d 100644
--- a/index.rst
+++ b/index.rst
@@ -123,6 +123,7 @@
    overview/lab_setup.rst
    overview/troubleshooting.rst
    overview/releases.rst
+   overview/contributing.rst
 
 .. toctree::
    :maxdepth: 1