DOC: Update contributing guide [ci skip]
DOC: Update contributing guide [ci skip]

dhomeier/astropy,mhvk/astropy,dhomeier/astropy,saimn/astropy,lpsinger/astropy,pllim/astropy,pllim/astropy,larrybradley/astropy,lpsinger/astropy,dhomeier/astropy,aleksandr-bakanov/astropy,mhvk/astropy,mhvk/astropy,mhvk/astropy,larrybradley/astropy,astropy/astropy,aleksandr-bakanov/astropy,astropy/astropy,saimn/astropy,dhomeier/astropy,larrybradley/astropy,mhvk/astropy,pllim/astropy,saimn/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,astropy/astropy,saimn/astropy,pllim/astropy,pllim/astropy,lpsinger/astropy,larrybradley/astropy,astropy/astropy,saimn/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,larrybradley/astropy,lpsinger/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,astropy/astropy,lpsinger/astropy