Add rule for verbose map key-value syntax

Author: christopheradams

README.md

@@ -1197,6 +1197,18 @@ directives (see [Modules](#modules)).
  %{a: 1, b: 2, c: 3}
  ```
 
+* <a name="map-key-arrow"></a>
+ Use the verbose key-value syntax for maps if any key is not an atom.
+ <sup>[[link](#map-key-arrow)]</sup>
+
+ ```elixir
+ # not preferred
+ %{"c" => 0, a: 1, b: 2}
+
+ # preferred
+ %{:a => 1, :b => 2, "c" => 0}
+ ```
+
 ### Strings
 
 * <a name="strings-matching-with-concatenator"></a>