Add a warning for enum elements with 'not_' prefix.

When a enum element contains the prefix 'not_'. I warns to users
to be aware of this new feature.

Example code:
    class Foo < ActiveRecord::Base
      enum status: [:sent, :not_sent]
    end

1 files changed, 8 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 8642227a2b..9e32e18e11 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,11 @@
+*   Add a warning for enum elements with 'not_' prefix.
+
+        class Foo
+          enum status: [:sent, :not_sent]
+        end
+
+    *Edu Depetris*
+
 *   Loading the schema for a model that has no `table_name` raises a `TableNotSpecified` error.
 
     *Guilherme Mansur*, *Eugene Kenny*