Add note about backport label
Add note about backport label

saimn/astropy,saimn/astropy,saimn/astropy,lpsinger/astropy,lpsinger/astropy,astropy/astropy,mhvk/astropy,mhvk/astropy,StuartLittlefair/astropy,pllim/astropy,mhvk/astropy,mhvk/astropy,larrybradley/astropy,pllim/astropy,mhvk/astropy,astropy/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,astropy/astropy,astropy/astropy,larrybradley/astropy,astropy/astropy,saimn/astropy,lpsinger/astropy,pllim/astropy,lpsinger/astropy,StuartLittlefair/astropy,larrybradley/astropy,pllim/astropy,larrybradley/astropy,larrybradley/astropy,pllim/astropy,lpsinger/astropy,StuartLittlefair/astropy,saimn/astropy