Add upload files documentation example

Author: Jeremiah VALERIE

Resources/doc/definitions/upload-files.md

@@ -0,0 +1,58 @@
+### Upload files
+
+here an example of how uploading can be done using this bundle
+
+* First define schema
+    ```yaml
+    RootMutation:
+        type: object
+        config:
+            fields:
+                uploadFile:
+                    builder: "Relay::Mutation"
+                    builderConfig:
+                        inputType: UploadFileInput
+                        payloadType: UploadFilePayload
+                        mutateAndGetPayload: '@=mutation("App\\GraphQL\\Mutation\\UploadMutation", [serv("request_stack"), value["title"]])'
+    
+    UploadFilePayload:
+        type: relay-mutation-payload
+        config:
+            fields:
+                files: {type: "[String!]!" }
+    
+    UploadFileInput:
+        type: relay-mutation-input
+        config:
+            fields:
+                title: {type: "String!"}
+    ```
+
+* here what the mutation look like
+
+    ```php
+    <?php
+    
+    namespace App\GraphQL\Mutation;
+    
+    use Overblog\GraphQLBundle\Error\UserError;
+    use Symfony\Component\HttpFoundation\Request;
+    
+    class UploadMutation
+    {
+        public function __invoke(Request $request, $title)
+        {
+            /** @var \Symfony\Component\HttpFoundation\FileBag; $requestFiles */
+            $requestFiles = $request->files;
+            if (!$requestFiles->has('myFile')) {
+                throw new UserError('File "myFile" is required.');
+            }
+            /** @var \Symfony\Component\HttpFoundation\File\UploadedFile $file */
+            $file = $requestFiles->get('myFile');
+     
+            // here do some work on your uploaded file
+    
+            return [$title];
+        }
+    }
+    ```