Added instructions to run  the App Extension API from master in the README.
Added instructions to run  the App Extension API from master in the README.

Also made some minor changes such as making the distinction between the "1Password extension" and the "1Password App Extension API"

ghecho/onepassword-app-extension,benjaminsnorris/onepassword-app-extension,ello/onepassword-app-extension,timonus/onepassword-app-extension,benjaminsnorris/onepassword-app-extension,losthvn/onepassword-app-extension,ello/onepassword-app-extension,DeskConnect/onepassword-app-extension,tapi/onepassword-app-extension,ello/onepassword-app-extension,losthvn/onepassword-app-extension,jonesgithub/onepassword-app-extension,Kingside/onepassword-app-extension,benjaminsnorris/onepassword-app-extension,AgileBits/onepassword-app-extension,remypanicker/onepassword-app-extension,myell0w/onepassword-app-extension,mro/onepassword-app-extension,hsk945/onepassword-app-extension,jonesgithub/onepassword-app-extension,AgileBits/onepassword-app-extension,ministrycentered/onepassword-app-extension,JaviSoto/onepassword-app-extension,remypanicker/onepassword-app-extension,panicinc/onepassword-app-extension,Kingside/onepassword-app-extension,ghecho/onepassword-app-extension,hsk945/onepassword-app-extension,PorterHoskins/onepassword-app-extension,DeskConnect/onepassword-app-extension,timonus/onepassword-app-extension,myell0w/onepassword-app-extension,Kingside/onepassword-app-extension,remypanicker/onepassword-app-extension,timonus/onepassword-app-extension,AgileBits/onepassword-app-extension,danielctull-forks/onepassword-app-extension,JaviSoto/onepassword-app-extension,ghecho/onepassword-app-extension,PorterHoskins/onepassword-app-extension,mro/onepassword-app-extension,panicinc/onepassword-app-extension