add section on using HTTPS in dev (#3048)
add section on using HTTPS in dev (#3048)

* add section on HTTPS in dev

* remove spaced periods

* fix Erlang doc link

* singular certificate

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