Clarify stable branch process in contributor documentation
Clarify stable branch process in contributor documentation

Explicitly mention that changes known to be needed for a specific
release should be pushed for review on that release's stable branch.

Change-Id: I5b90edb2635495493639e181741eea111ca30d7b

qtproject/qtqa-gerrit,MerritCR/merrit,Team-OctOS/host_gerrit,GerritCodeReview/gerrit,thinkernel/gerrit,joshuawilson/merrit,renchaorevee/gerrit,qtproject/qtqa-gerrit,gerrit-review/gerrit,quyixia/gerrit,gcoders/gerrit,dwhipstock/gerrit,thesamet/gerrit,joshuawilson/merrit,midnightradio/gerrit,supriyantomaftuh/gerrit,qtproject/qtqa-gerrit,bpollack/gerrit,Distrotech/gerrit,pkdevbox/gerrit,gracefullife/gerrit,Saulis/gerrit,pkdevbox/gerrit,MerritCR/merrit,gerrit-review/gerrit,netroby/gerrit,zommarin/gerrit,gcoders/gerrit,Overruler/gerrit,quyixia/gerrit,thesamet/gerrit,gerrit-review/gerrit,anminhsu/gerrit,midnightradio/gerrit,bpollack/gerrit,netroby/gerrit,renchaorevee/gerrit,gcoders/gerrit,netroby/gerrit,Team-OctOS/host_gerrit,MerritCR/merrit,Team-OctOS/host_gerrit,gerrit-review/gerrit,zommarin/gerrit,zommarin/gerrit,gcoders/gerrit,Seinlin/gerrit,bpollack/gerrit,pkdevbox/gerrit,gcoders/gerrit,dwhipstock/gerrit,Saulis/gerrit,MerritCR/merrit,bpollack/gerrit,bootstraponline-archive/gerrit-mirror,Distrotech/gerrit,TonyChai24/test,bpollack/gerrit,thinkernel/gerrit,Team-OctOS/host_gerrit,Seinlin/gerrit,TonyChai24/test,midnightradio/gerrit,Saulis/gerrit,Seinlin/gerrit,Overruler/gerrit,anminhsu/gerrit,MerritCR/merrit,jackminicloud/test,bootstraponline-archive/gerrit-mirror,supriyantomaftuh/gerrit,thinkernel/gerrit,anminhsu/gerrit,Seinlin/gerrit,Overruler/gerrit,dwhipstock/gerrit,WANdisco/gerrit,hdost/gerrit,dwhipstock/gerrit,pkdevbox/gerrit,zommarin/gerrit,Overruler/gerrit,joshuawilson/merrit,WANdisco/gerrit,midnightradio/gerrit,WANdisco/gerrit,hdost/gerrit,netroby/gerrit,thinkernel/gerrit,quyixia/gerrit,Team-OctOS/host_gerrit,Team-OctOS/host_gerrit,hdost/gerrit,gracefullife/gerrit,jackminicloud/test,gcoders/gerrit,anminhsu/gerrit,supriyantomaftuh/gerrit,TonyChai24/test,gcoders/gerrit,gerrit-review/gerrit,GerritCodeReview/gerrit,renchaorevee/gerrit,gracefullife/gerrit,WANdisco/gerrit,WANdisco/gerrit,thinkernel/gerrit,joshuawilson/merrit,supriyantomaftuh/gerrit,dwhipstock/gerrit,Saulis/gerrit,qtproject/qtqa-gerrit,midnightradio/gerrit,GerritCodeReview/gerrit,thesamet/gerrit,MerritCR/merrit,anminhsu/gerrit,dwhipstock/gerrit,hdost/gerrit,bpollack/gerrit,gerrit-review/gerrit,renchaorevee/gerrit,Saulis/gerrit,bootstraponline-archive/gerrit-mirror,thesamet/gerrit,GerritCodeReview/gerrit,Distrotech/gerrit,bootstraponline-archive/gerrit-mirror,bootstraponline-archive/gerrit-mirror,joshuawilson/merrit,quyixia/gerrit,anminhsu/gerrit,gracefullife/gerrit,pkdevbox/gerrit,Seinlin/gerrit,jackminicloud/test,MerritCR/merrit,joshuawilson/merrit,renchaorevee/gerrit,Team-OctOS/host_gerrit,Distrotech/gerrit,supriyantomaftuh/gerrit,Seinlin/gerrit,Seinlin/gerrit,Overruler/gerrit,quyixia/gerrit,Distrotech/gerrit,Saulis/gerrit,joshuawilson/merrit,quyixia/gerrit,netroby/gerrit,jackminicloud/test,thesamet/gerrit,dwhipstock/gerrit,thinkernel/gerrit,Distrotech/gerrit,supriyantomaftuh/gerrit,renchaorevee/gerrit,thinkernel/gerrit,qtproject/qtqa-gerrit,jackminicloud/test,supriyantomaftuh/gerrit,WANdisco/gerrit,TonyChai24/test,GerritCodeReview/gerrit,renchaorevee/gerrit,hdost/gerrit,thesamet/gerrit,Overruler/gerrit,WANdisco/gerrit,TonyChai24/test,thesamet/gerrit,anminhsu/gerrit,MerritCR/merrit,TonyChai24/test,zommarin/gerrit,joshuawilson/merrit,gracefullife/gerrit,jackminicloud/test,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,jackminicloud/test,midnightradio/gerrit,pkdevbox/gerrit,GerritCodeReview/gerrit,TonyChai24/test,netroby/gerrit,gerrit-review/gerrit,hdost/gerrit,qtproject/qtqa-gerrit,quyixia/gerrit,hdost/gerrit,netroby/gerrit,pkdevbox/gerrit,bootstraponline-archive/gerrit-mirror,Distrotech/gerrit