Update Contributing blurb in the docs.
Update Contributing blurb in the docs.

Contributing blurb previously mentioned using Github pull requests.
Contributions should go to Gerrit instead.

Change-Id: Id7b7f968fa5a3785c19704b4c1c2408f48b335ca

guptaankita/python-novaclient,citrix-openstack-build/python-novaclient,Shihta/python-novaclient,Shihta/python-novaclient,JioCloud/python-novaclient,takeshineshiro/python-novaclient,mat128/python-novaclient,metacloud/python-novaclient,JioCloud/python-novaclient,CCI-MOC/python-novaclient,akash1808/python-novaclient,metacloud/python-novaclient,akash1808/python-novaclient,CCI-MOC/python-novaclient,mat128/python-novaclient,ntt-sic/python-novaclient,guptaankita/python-novaclient,citrix-openstack-build/python-novaclient,takeshineshiro/python-novaclient,ntt-sic/python-novaclient