JavaScript use policy

Add a point to HACKING.rst pointing to the Horizon docs about
contributing.

Add information about JavaScript support policy to the contributing
docs.

The policy is based on:
http://lists.openstack.org/pipermail/openstack-dev/2013-November/020342.html

Change-Id: I16a51c7ab7faf68d6e7984c113da175c5c08650d
Merge "JavaScript use policy"