Add optional 'extensions' entry to errors

spec/Section 7 -- Response.md

@@ -248,6 +248,45 @@ be the same:
 }
 ```
 
-GraphQL servers may provide additional entries to error as they choose to
-produce more helpful or machine-readable errors, however future versions of the
-spec may describe additional entries to errors.
+GraphQL services may provide an additional entry to errors with key `extensions`.
+This entry, if set, must have a map as its value. This entry is reserved for
+implementors to add additional information to errors however they see fit, and
+there are no additional restrictions on its contents.
+
+```json example
+{
+  "errors": [
+    {
+      "message": "Name for character with ID 1002 could not be fetched.",
+      "locations": [ { "line": 6, "column": 7 } ],
+      "path": [ "hero", "heroFriends", 1, "name" ],
+      "extensions": {
+        "code": "CAN_NOT_FETCH_BY_ID",
+        "timestamp": "Fri Feb 9 14:33:09 UTC 2018"
+      }
+    }
+  ]
+}
+```
+
+GraphQL services should not provide any additional entries to the error format
+since they could conflict with additional entries that may be added in future
+versions of this specification.
+
+> Note: Previous versions of this spec did not describe the `extensions` entry
+> for error formatting. While non-specified entries are not violations, they are
+> still discouraged.
+
+```json counter-example
+{
+  "errors": [
+    {
+      "message": "Name for character with ID 1002 could not be fetched.",
+      "locations": [ { "line": 6, "column": 7 } ],
+      "path": [ "hero", "heroFriends", 1, "name" ]
+      "code": "CAN_NOT_FETCH_BY_ID",
+      "timestamp": "Fri Feb 9 14:33:09 UTC 2018"
+    }
+  ]
+}
+```