Update RELEASE.md to mention pushing installer to hex
Update RELEASE.md to mention pushing installer to hex

eksperimental/phoenix,rafbgarcia/phoenix,0x00evil/phoenix,Joe-noh/phoenix,Gazler/phoenix,Joe-noh/phoenix,phoenixframework/phoenix,Gazler/phoenix,jwarwick/phoenix,nshafer/phoenix,phoenixframework/phoenix,mitchellhenke/phoenix,0x00evil/phoenix,mitchellhenke/phoenix,nshafer/phoenix,keathley/phoenix,nshafer/phoenix,rafbgarcia/phoenix,evax/phoenix,eksperimental/phoenix,phoenixframework/phoenix,rafbgarcia/phoenix,0x00evil/phoenix,dmarkow/phoenix,optikfluffel/phoenix,optikfluffel/phoenix,keathley/phoenix,Kosmas/phoenix,jwarwick/phoenix,Kosmas/phoenix,michalmuskala/phoenix,michalmuskala/phoenix,optikfluffel/phoenix,evax/phoenix,dmarkow/phoenix