Add note in dev docs about PLY
Add note in dev docs about PLY

[docs only]

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