Add a section about mutability to README.
Add a section about mutability to README.

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