Added printIntrospectionSchema to exports (#905)

Docs at http://graphql.org/graphql-js/utilities/ say "You can import either from the graphql/utilities module, or from the root graphql module" and lists printIntrospectionSchema, yet it is not exported via the root module.

Author: aeiz

src/index.js

@@ -319,6 +319,10 @@ export {
   // Print a GraphQLSchema to GraphQL Schema language.
   printSchema,
 
+  // Prints the built-in introspection schema in the Schema Language
+  // format.
+  printIntrospectionSchema,
+
   // Print a GraphQLType to GraphQL Schema language.
   printType,