utils: add a note in docs on how to deal with certificate errors
utils: add a note in docs on how to deal with certificate errors

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