add initial outline for field extensions

spec/Appendix C -- Grammar Summary.md

@@ -318,6 +318,12 @@ FieldsDefinition : { FieldDefinition+ }
 FieldDefinition : Description? Name ArgumentsDefinition? : Type
 Directives[Const]?
 
+FieldExtension :
+
+- extend field MemberCoordinate ImplementsInterfaces? Directives[const]?
+  [lookahead != `{`]
+- extend field description MemberCoordinate [lookahead != `{`]
+
 ArgumentsDefinition : ( InputValueDefinition+ )
 
 InputValueDefinition : Description? Name : Type DefaultValue? Directives[Const]?
@@ -329,6 +335,10 @@ InterfaceTypeDefinition :
 - Description? interface Name ImplementsInterfaces? Directives[Const]?
   [lookahead != `{`]
 
+InterfaceFieldExtension :
+
+- extend interface MemberCoordinate Directives[const]?
+
 InterfaceTypeExtension :
 
 - extend interface Name ImplementsInterfaces? Directives[Const]?

spec/Section 3 -- Type System.md

@@ -1105,6 +1105,36 @@ Object type extensions have the potential to be invalid if incorrectly defined.
 6. The resulting extended object type must be a super-set of all interfaces it
    implements.
 
+### Field Extensions
+
+FieldExtension :
+
+- extend field MemberCoordinate ImplementsInterfaces? Directives[const]?
+  [lookahead != `{`]
+- extend field description MemberCoordinate [lookahead != `{`]
+
+Field extensions are used to represent a field which has been extended from some
+previously defined field. For example this may be a GraphQL service which is
+itself an extension of another GraphQL service.
+
+In this example, we can deprecate the id field on the User type.
+
+```graphql example
+extend field User.name @deprecated(”Some reason”)
+```
+
+** Field Validation **
+
+Field validation have the potential to be invalid if incorrectly defined.
+
+1. The field must already be defined and a field for a Type
+2. Any non-repeatable directives provided must not already apply to the previous
+   field.
+3. Any interfaces provided must not be already implemented by the previous
+   field.
+4. The resulting extended field must be a super-set of all interfaces it
+   implements.
+
 ## Interfaces
 
 InterfaceTypeDefinition :
@@ -1358,6 +1388,34 @@ defined.
 6. The resulting extended Interface type must be a super-set of all Interfaces
    it implements.
 
+#### Interface Field Extensions
+
+InterfaceFieldExtension :
+
+- extend interface MemberCoordinate Directives[const]?
+
+Interface field extensions are used to represent an interface which has been
+extended from some previous interface. For example this may be a GraphQL service
+which is itself an extension of another GraphQL service.
+
+In this example, the name field on the User type is deprecated for an interface.
+
+```graphql example
+extend interface User.name @deprecated("Some reason.")
+```
+
+** Field Validation **
+
+Field validation have the potential to be invalid if incorrectly defined.
+
+1. The field must already be defined and a field for a Type
+2. Any non-repeatable directives provided must not already apply to the previous
+   field.
+3. Any interfaces provided must not be already implemented by the previous
+   field.
+4. The resulting extended field must be a super-set of all interfaces it
+   implements.
+
 ## Unions
 
 UnionTypeDefinition : Description? union Name Directives[Const]?