Improve RELEASE instructions (#4254)
Improve RELEASE instructions (#4254)

It mentions specifically that `mix test` needs to be run for Phoenix and the Phoenix installer.
nshafer/phoenix,0x00evil/phoenix,phoenixframework/phoenix,rafbgarcia/phoenix,0x00evil/phoenix,rafbgarcia/phoenix,optikfluffel/phoenix,phoenixframework/phoenix,rafbgarcia/phoenix,eksperimental/phoenix,optikfluffel/phoenix,nshafer/phoenix,phoenixframework/phoenix,eksperimental/phoenix,optikfluffel/phoenix,nshafer/phoenix,0x00evil/phoenix