Add section for hash syntax of enums

Closes rubocop#144

Ref: rubocop/rubocop-rails#78

README.adoc

@@ -385,6 +385,24 @@ class Transaction < ActiveRecord::Base
 end
 ----
 
+=== Enums [[enums]]
+
+Prefer hash syntax over arrays for `enums`.
+
+[source,ruby]
+----
+class Transaction < ActiveRecord::Base
+  # bad - implicit values - ordering in array matters
+  enum type: %i[credit debit]
+
+  # good - explicit values - ordering does not matter
+  enum type: {
+    credit: 0,
+    debit: 1,
+  }
+end
+----
+
 === Macro Style Methods [[macro-style-methods]]
 
 Group macro-style methods (`has_many`, `validates`, etc) in the beginning of the class definition.