update .env.example to have all vars and add more comments
update .env.example to have all vars and add more comments

rickmak/skygear-server,SkygearIO/skygear-server,SkygearIO/skygear-server,SkygearIO/skygear-server,SkygearIO/skygear-server,rickmak/skygear-server,rickmak/skygear-server