Add instructions for adding GraphiQL assets to manifest.js

These instructions are needed as Sprockets 4 changes the logic for which
assets are to be included for precompilation by default. They can be
removed once rmosolgo/graphiql-rails#76 is
merged and released.

content/backend/graphql-ruby/2-queries.md

@@ -107,7 +107,12 @@ Fields can be resolved in one of two ways:
 
 It's time to check what you've done so far! For this, you'll use [GraphiQL](https://github.com/graphql/graphiql), an in-browser IDE for running GraphQL queries.
 
-GraphiQL had already been added to your application when you executed `rails generate graphql:install` in the terminal before, so you don't have to do any extra work to set it up.
+GraphiQL had already been added to your application when you executed `rails generate graphql:install` in the terminal before, but you have tell the asset pipeline to precompile its assets. Add this at the end of `/app/assets/config/manifest.js`:
+
+```js
+//= link graphiql/rails/application.css
+//= link graphiql/rails/application.js
+```
 
 Open your browser at http://localhost:3000/graphiql