docs: Move tip about Extension Drivers into misc section
docs: Move tip about Extension Drivers into misc section

Change-Id: Ifd458495992a0fd1b7437c315b4fe45906830cb1

pombredanne/devstack,dtroyer/devstack,bswartz/devstack,nawawi/openstack,thomasem/devstack,dirkmueller/devstack,richliu/devstack,Mahito/devstack,dtroyer/devstack,mssumanth/devstack,Mahito/devstack,bigswitch/devstack,prithivm/branch01,avvocatodemarchis/devstack,varunarya10/devstack,noironetworks/devstack,prithivm/branch01,avvocatodemarchis/devstack,pombredanne/devstack,ozamiatin/devstack,nawawi/openstack,noironetworks/devstack,richliu/devstack,dirkmueller/devstack,bswartz/devstack,ozamiatin/devstack,bigswitch/devstack,samgoon/devstack,openstack-dev/devstack,group-policy/devstack,varunarya10/devstack,samgoon/devstack,mssumanth/devstack,group-policy/devstack,thomasem/devstack,openstack-dev/devstack