Add guideline for @SPEC + @doc

Author: moxley

README.md

@@ -1073,10 +1073,14 @@ directives (see [Modules](#modules)).
 
 * <a name="spec-spacing"></a>
  Place specifications right before the function definition,
+ after the `@doc`,
  without separating them by a blank line.
  <sup>[[link](#spec-spacing)]</sup>
 
  ```elixir
+ @doc """
+ Some function description.
+ """
  @spec some_function(term) :: result
  def some_function(some_data) do
  {:ok, some_data}