Add a note on how to reference a callback
Add a note on how to reference a callback

gfvcastro/elixir,elixir-lang/elixir,ggcampinho/elixir,pedrosnk/elixir,lexmag/elixir,joshprice/elixir,gfvcastro/elixir,kimshrier/elixir,ggcampinho/elixir,antipax/elixir,beedub/elixir,beedub/elixir,pedrosnk/elixir,kelvinst/elixir,kimshrier/elixir,michalmuskala/elixir,lexmag/elixir,antipax/elixir,kelvinst/elixir