Add description on how to run ./run_test.sh -8
Add description on how to run ./run_test.sh -8

./run_test.sh was deprecated and I used to
run './run_test.sh -8' to only check changed files instead
of whole nova tree. Add this hint 'tox -e pep8 -- -HEAD'
to help people who don't know this.

Change-Id: I0408d0249064e1db89d17c19a524b43a46159a24

bigswitch/nova,sebrandon1/nova,openstack/nova,rajalokan/nova,vmturbo/nova,cloudbase/nova,hanlind/nova,NeCTAR-RC/nova,CEG-FYP-OpenStack/scheduler,rajalokan/nova,gooddata/openstack-nova,dims/nova,klmitch/nova,mikalstill/nova,vmturbo/nova,Juniper/nova,rajalokan/nova,BeyondTheClouds/nova,cloudbase/nova,phenoxim/nova,cyx1231st/nova,openstack/nova,rahulunair/nova,zhimin711/nova,hanlind/nova,jianghuaw/nova,klmitch/nova,mahak/nova,alaski/nova,sebrandon1/nova,NeCTAR-RC/nova,sebrandon1/nova,cloudbase/nova,phenoxim/nova,mahak/nova,cernops/nova,bigswitch/nova,mikalstill/nova,vmturbo/nova,cernops/nova,rahulunair/nova,jianghuaw/nova,dims/nova,hanlind/nova,Juniper/nova,Juniper/nova,klmitch/nova,klmitch/nova,vmturbo/nova,CEG-FYP-OpenStack/scheduler,jianghuaw/nova,BeyondTheClouds/nova,cyx1231st/nova,mikalstill/nova,gooddata/openstack-nova,mahak/nova,gooddata/openstack-nova,rahulunair/nova,rajalokan/nova,cernops/nova,Juniper/nova,openstack/nova,BeyondTheClouds/nova,gooddata/openstack-nova,jianghuaw/nova,zhimin711/nova,alaski/nova