Update README to mention `gulp bundle` (#8586)
Update README to mention `gulp bundle` (#8586)


HuddledMasses/Prebid.js,HuddledMasses/Prebid.js,ImproveDigital/Prebid.js,PubWise/Prebid.js,gumgum/Prebid.js,prebid/Prebid.js,gumgum/Prebid.js,ImproveDigital/Prebid.js,PubWise/Prebid.js,prebid/Prebid.js,gumgum/Prebid.js,prebid/Prebid.js,HuddledMasses/Prebid.js,ImproveDigital/Prebid.js,PubWise/Prebid.js