Add GraphQL Docs

Author: m1guelpf

doc/graphql.md

@@ -0,0 +1,10 @@
+## GraphQL API
+[Back to the navigation](README.md)
+
+Wraps [GitHub v4 API (GraphQL API)](http://developer.github.com/v4/).
+
+#### Execute a query
+
+```php
+$rateLimits = $client->api('graphql')->execute($query);
+```