Add how to define GraphQL enum from Rails enum

doc/development/api_graphql_styleguide.md

@@ -486,6 +486,28 @@ end
 Enum values can be deprecated using the
 [`deprecated` keyword](#deprecating-fields-and-enum-values).
 
+### Defining GraphQL enums dynamically from Rails enums
+
+If your GraphQL enum is backed by a [Rails enum](creating_enums.md), then consider
+using the Rails enum to dynamically define the GraphQL enum values. Doing so
+binds the GraphQL enum values to the Rails enum definition, so if values are
+ever added to the Rails enum then the GraphQL enum automatically reflects the change.
+
+Example:
+
+```ruby
+module Types
+  class IssuableSeverityEnum < BaseEnum
+    graphql_name 'IssuableSeverity'
+    description 'Incident severity'
+
+    ::IssuableSeverity.severities.keys.each do |severity|
+      value severity.upcase, value: severity, description: "#{severity.titleize} severity"
+    end
+  end
+end
+```
+
 ## JSON
 
 When data to be returned by GraphQL is stored as