Add bit about documentation style to CONTRIBUTING
Add bit about documentation style to CONTRIBUTING

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