Add graphiql custom http headers documentation

Resources/doc/definitions/graphiql/custom-http-headers.md

@@ -0,0 +1,50 @@
+Custom HTTP headers
+==============
+
+GraphiQL, provided by this bundle, sends the following default headers on each request:
+
+```js
+headers = {
+  "Accept": "application/json",
+  "Content-Type": "application/json"
+};
+```
+
+Headers sent by GraphiQL can be modified. 
+For example, let's assume an `access-token` header is required in development.
+The header can be added the following way:
+
+1. Override the default GraphiQL template:
+
+```yml
+# app/config/config_dev.yml
+overblog_graphql:
+  templates:
+    graphiql: AppBundle:GraphiQL:index.html.twig
+```
+2. Create a new template:  
+
+```twig
+{# src/AppBundle/Resources/views/GraphiQL/index.html.twig #}
+{% extends 'OverblogGraphQLBundle:GraphiQL:index.html.twig' %}
+
+{% block graphql_fetcher_headers %}
+headers = {
+  "Accept": "application/json",
+  "Content-Type": "application/json",
+  "access-token": "sometoken"
+};
+{% endblock graphql_fetcher_headers %}
+```
+
+Or append headers instead of replacing the default one:
+
+```twig
+{# src/AppBundle/Resources/views/GraphiQL/index.html.twig #}
+{% extends 'OverblogGraphQLBundle:GraphiQL:index.html.twig' %}
+
+{% block graphql_fetcher_headers %}
+{{ parent() }}
+headers["access-token"] = "sometoken";
+{% endblock graphql_fetcher_headers %}
+```

Resources/doc/definitions/graphiql/index.md

@@ -0,0 +1,4 @@
+GraphiQL
+========
+
+* [Custom HTTP headers](custom-http-headers.md)

Resources/doc/definitions/index.md

@@ -13,5 +13,6 @@ Go further
 * [Builders](builders/index.md)
 * [Expression language](expression-language.md)
 * [Debug](debug/index.md)
+* [GraphiQL](graphiql/index.md)
 
 Next step [Data fetching](../data-fetching/index.md).