doc: update threading.rst
doc: update threading.rst

There are two ways to give other greenthread chance to run:
greenthread.sleep(0) or time.sleep(0). Add the second way in
threading.rst and recommend the second way for contributors.

Change-Id: I33dc73a0edfa5fde0e5fa8051a594874ab166970

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