feat(graphiql-fetcher): added note regarding subscription detection

src/graphiql-fetcher.ts

@@ -1,6 +1,10 @@
 import { SubscriptionClient } from './client';
 
 const hasSubscriptionOperation = (graphQlParams: any) => {
+  // We are using this method instead using GraphQL parser, because GraphiQL
+  // does not expose GraphQL, and we don't want to load it as dependency in the transport,
+  // in order to reduce it's client-side package size. 
+  // Also, GraphQL does not have any CDN version that can be loaded directly.
   const query = graphQlParams.query;
   const operationName = graphQlParams.operationName;