update readme to reflect the arm64 addition and iOS profile builds not being built by default
update readme to reflect the arm64 addition and iOS profile builds not being built by default

adozenlines/webrtc-build-scripts,huynguyen/webrtc-build-scripts,hareshsandeep/webrtc-build-scripts,imReker/webrtc-build-scripts,nehaysingh/webrtc-build-scripts,zero1zero/webrtc-build-scripts,zzhhui/webrtc-build-scripts,act262/webrtc-build-scripts,imReker/webrtc-build-scripts,hareshsandeep/webrtc-build-scripts,ShengQiangLiu/webrtc-build-scripts,czhao/webrtc-build-scripts,lulufei/webrtc-build-scripts,huynguyen/webrtc-build-scripts,nehaysingh/webrtc-build-scripts,pristineio/webrtc-build-scripts,ShengQiangLiu/webrtc-build-scripts,aditya-agarwal/webrtc-build-scripts,zzhhui/webrtc-build-scripts,shelsonjava/webrtc-build-scripts,hareshsandeep/webrtc-build-scripts,pristineio/webrtc-build-scripts,act262/webrtc-build-scripts,aditya-agarwal/webrtc-build-scripts,liushimeng/webrtc-build-scripts,zzhhui/webrtc-build-scripts,shelsonjava/webrtc-build-scripts,GabrielCastro/webrtc-build-scripts,liushimeng/webrtc-build-scripts,adozenlines/webrtc-build-scripts,sundeepdev/webrtc-build-scripts,liushimeng/webrtc-build-scripts,GabrielCastro/webrtc-build-scripts,sundeepdev/webrtc-build-scripts,contra/webrtc-build-scripts,GabrielCastro/webrtc-build-scripts,huynguyen/webrtc-build-scripts,zero1zero/webrtc-build-scripts,contra/webrtc-build-scripts,adozenlines/webrtc-build-scripts,sundeepdev/webrtc-build-scripts,ShengQiangLiu/webrtc-build-scripts,czhao/webrtc-build-scripts,nehaysingh/webrtc-build-scripts,imReker/webrtc-build-scripts,lulufei/webrtc-build-scripts,zero1zero/webrtc-build-scripts,shelsonjava/webrtc-build-scripts,lulufei/webrtc-build-scripts,pristineio/webrtc-build-scripts,contra/webrtc-build-scripts,czhao/webrtc-build-scripts,aditya-agarwal/webrtc-build-scripts,act262/webrtc-build-scripts