[docs] Emphasize netjsonconfig is engine of openwisp2

Author: nemesifier

README.rst

@@ -16,7 +16,8 @@ netjsonconfig
 
 ------------
 
-Netjsonconfig is part of the `OpenWISP project <http://openwrt.org>`_.
+Netjsonconfig is part of the `OpenWISP project <http://openwrt.org>`_ and it's the official
+configuration engine of `OpenWISP 2 <https://github.com/openwisp/ansible-openwisp2>`_.
 
 .. image:: http://netjsonconfig.openwisp.org/en/latest/_images/openwisp.org.svg
   :target: http://openwisp.org

docs/source/index.rst

@@ -15,7 +15,8 @@ netjsonconfig
 .. image:: https://badge.fury.io/py/netjsonconfig.svg
   :target: http://badge.fury.io/py/netjsonconfig
 
-Netjsonconfig is part of the `OpenWISP project <http://openwisp.org>`_.
+Netjsonconfig is part of the `OpenWISP project <http://openwrt.org>`_ and it's the official
+configuration engine of `OpenWISP 2 <https://github.com/openwisp/ansible-openwisp2>`_.
 
 .. image:: ./images/openwisp.org.svg
   :target: http://openwisp.org