Document Directives - grammar plus description

Author: langpavel

spec/Appendix B -- Grammar Summary.md

@@ -94,10 +94,14 @@ lines and uniform indentation with {BlockStringValue()}.
 Document : Definition+
 
 Definition :
+  - DocumentDirective
   - ExecutableDefinition
   - TypeSystemDefinition
   - TypeSystemExtension
 
+DocumentDirective :
+  - Directive[Const]
+
 ExecutableDefinition :
   - OperationDefinition
   - FragmentDefinition

spec/Section 2 -- Language.md

@@ -172,6 +172,7 @@ characters to support interoperation with as many other systems as possible.
 Document : Definition+
 
 Definition :
+  - DocumentDirective
   - ExecutableDefinition
   - TypeSystemDefinition
   - TypeSystemExtension
@@ -1094,3 +1095,24 @@ and operations.
 
 As future versions of GraphQL adopt new configurable execution capabilities,
 they may be exposed via directives.
+
+
+## Document Directives
+
+DocumentDirective :
+  - Directive[Const]
+
+{DocumentDirective} is special kind of {Definition} and should be handled
+on AST level by implementation in similar way like {TypeSystemDefinition}
+or {ExecutableDefinition}.
+
+Documents of any kind may contain zero or more {DocumentDirective}s.
+Same directive may be repeated multiple times at document level:
+
+```graphql example
+@import(from: "./common")
+@import(from: "./user")
+```
+
+Unknown directives may be ignored, but it is recommended for services to fail
+if directive is not recognized.