allow directives on directive definitions

Author: benhead

spec/Section 3 -- Type System.md

@@ -1607,7 +1607,7 @@ Expected Type | Internal Value   | Coerced Result
 
 ## Directives
 
-DirectiveDefinition : Description? directive @ Name ArgumentsDefinition? on DirectiveLocations
+DirectiveDefinition : Description? directive @ Name ArgumentsDefinition? on DirectiveLocations Directives[Const]?
 
 DirectiveLocations :
   - `|`? DirectiveLocation
@@ -1639,6 +1639,7 @@ TypeSystemDirectiveLocation : one of
   `ENUM_VALUE`
   `INPUT_OBJECT`
   `INPUT_FIELD_DEFINITION`
+  `DIRECTIVE_DEFINITION`
 
 A GraphQL schema describes directives which are used to annotate various parts
 of a GraphQL document as an indicator that they should be evaluated differently