Add rule for map key shorthand

Closes #169.

Author: christopheradams

README.md

@@ -1185,6 +1185,18 @@ directives (see [Modules](#modules)).
  some_value = [a: "baz", b: "qux"]
  ```
 
+* <a name="map-key-atom"></a>
+ Use the shorthand key-value syntax for maps when all of the keys are atoms.
+ <sup>[[link](#map-key-atom)]</sup>
+
+ ```elixir
+ # not preferred
+ %{:a => 1, :b => 2, :c => 0}
+
+ # preferred
+ %{a: 1, b: 2, c: 3}
+ ```
+
 ### Strings
 
 * <a name="strings-matching-with-concatenator"></a>