adding info about the validate callback being optional
adding info about the validate callback being optional

tpiecora/joi,danielb2/joi,opyate/joi,geminiyellow/joi,lloydbenson/joi,naitsric/joi,cesarmarinhorj/joi,rmoorman/joi,thethanghn/react-native-joi,ck-lee/joi,jagoda/joi,davidbenko9/joi,pikselpalette/joi,jeffbski/joi,bananaoomarang/joi,pandeysoni/joi,olistic/joi-extended,robertherber/joi-extended