Add perf. notes for convolve
Add perf. notes for convolve

Signed-off-by: James Noss <897e4a798cfa3e609063a099c3c4ac849e1a5839@stsci.edu>

saimn/astropy,stargaser/astropy,stargaser/astropy,mhvk/astropy,pllim/astropy,MSeifert04/astropy,dhomeier/astropy,dhomeier/astropy,MSeifert04/astropy,larrybradley/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,stargaser/astropy,astropy/astropy,StuartLittlefair/astropy,saimn/astropy,mhvk/astropy,MSeifert04/astropy,mhvk/astropy,larrybradley/astropy,StuartLittlefair/astropy,astropy/astropy,StuartLittlefair/astropy,larrybradley/astropy,StuartLittlefair/astropy,dhomeier/astropy,lpsinger/astropy,pllim/astropy,dhomeier/astropy,lpsinger/astropy,astropy/astropy,astropy/astropy,saimn/astropy,MSeifert04/astropy,bsipocz/astropy,mhvk/astropy,larrybradley/astropy,bsipocz/astropy,stargaser/astropy,saimn/astropy,bsipocz/astropy,dhomeier/astropy,lpsinger/astropy,pllim/astropy,mhvk/astropy,StuartLittlefair/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,aleksandr-bakanov/astropy,aleksandr-bakanov/astropy,bsipocz/astropy,pllim/astropy,astropy/astropy,saimn/astropy,pllim/astropy,larrybradley/astropy