Make root query operation type optional

This makes the root query operation type optional like the other types.

Schema introspection is unaffected. The `__schema` and `__type` fields were
already described as "implicit" and not part of the defined root query
operation type. A schema without a root query type therefore only supports
these queries.

Fixes graphql#490

Author: victorandree

spec/Section 3 -- Type System.md

@@ -69,7 +69,11 @@ A schema defines the initial root operation type for each kind of operation it
 supports: query, mutation, and subscription; this determines the place in the
 type system where those operations begin.
 
-The `query` root operation type must be provided and must be an Object type.
+A schema must define at least one root operation type.
+
+The `query` root operation type is optional; if it is not provided, the service
+does not support queries except schema introspection. If it is provided, it
+must be an Object type.
 
 The `mutation` root operation type is optional; if it is not provided, the
 service does not support mutations. If it is provided, it must be an

spec/Section 4 -- Introspection.md

@@ -111,14 +111,15 @@ __type(name: String!): __Type
 ```
 
 These fields are implicit and do not appear in the fields list in the root type
-of the query operation.
+of the query operation. They are available even if the schema does not provide
+a query operation of its own.
 
 The schema of the GraphQL schema introspection system:
 
 ```graphql
 type __Schema {
   types: [__Type!]!
-  queryType: __Type!
+  queryType: __Type
   mutationType: __Type
   subscriptionType: __Type
   directives: [__Directive!]!