link to style guide in CONTRIBUTING.md (#829)
link to style guide in CONTRIBUTING.md (#829)

When responding to [this issue](https://github.com/doomspork/elixir-school/issues/828) I realized the style guide I suggested to consult was not linked to in CONTRIBUTING.md, and figured this was a no-brainer to help new contributors get started and not have minor issues that slow the merge flow.
ElixirVietnam/elixir-school,huydx/elixir-school,nscyclone/elixir-school,erickgnavar/elixir-school,doomspork/elixir_school,huydx/elixir-school,nscyclone/elixir-school,ElixirVietnam/elixir-school,erickgnavar/elixir-school,doomspork/elixir-school,nscyclone/elixir-school,doomspork/elixir-school,doomspork/elixir_school,yuriibodarev/elixir-school,yuriibodarev/elixir-school,doomspork/elixir_school,doomspork/elixir-school,psantos10/elixir-school,psantos10/elixir-school,psantos10/elixir-school,nscyclone/elixir-school,erickgnavar/elixir-school,yuriibodarev/elixir-school,ElixirVietnam/elixir-school,huydx/elixir-school