Add graphiQL screenhsot

Author: fzaninotto

README.md

@@ -90,7 +90,9 @@ Go to http://localhost:3000/?query=query%20%7B%20Post(id%3A%201)%20%7Bid%20title
 
 The json-graphql-server accepts queries in GET and POST. Under the hood, it uses [Apollo's `graphql-server` module](http://dev.apollodata.com/tools/graphql-server/requests.html). Please refer to their documentations for details about passing variables, etc.
 
-Note that the server is [GraphiQL](https://github.com/skevy/graphiql-app/releases) enabled.
+Note that the server is [GraphiQL](https://github.com/skevy/graphiql-app/releases) enabled, so you can query your server using a full-featured graphical user interface, providing autosuggest, history, etc.
+
+![GraphiQL client using json-graphql-server](http://static.marmelab.com/graphiql-json.png)
 
 ## Install