Add link to Elixir School writing documentation to our guides

[joshsmith]

We should state that we prefer people to add documentation, and link to https://elixirschool.com/lessons/basics/documentation/ to show how to write Elixir docs.

For changes that affect the API endpoints directly, we should link to docs on writing API Blueprint documentation.

[joshsmith]

Should also link to:

• http://elixir-lang.org/docs/stable/elixir/Module.html
• http://elixir-lang.org/docs/master/elixir/writing-documentation.html