added description for the new field

Author: grundid

README.md

@@ -28,4 +28,48 @@ Since this package makes use of [file_picker package](https://pub.dev/packages/f
       print(val);
     },
   ),
+```
+
+On mobile platforms the file picker will open a default document picker if used with FileType.any. 
+If you want to be able to pick documents and images in the same form field you will need to define
+different file types and thus different selectors. To achieve this use the typeSelectors parameter:
+This way the user will see two buttons to open a file picker for documents and a file picker for the photos
+gallery.
+
+For example:
+
+```dart
+FormBuilderFilePicker(
+  name: "attachments",
+  previewImages: false,
+  allowMultiple: true,
+  withData: true,
+  typeSelectors: [
+    TypeSelector(
+      type: FileType.any,
+      selector: Row(
+        children: <Widget>[
+          Icon(Icons.add_circle),
+          Padding(
+            padding: const EdgeInsets.only(left: 8.0),
+            child: Text("Add documents"),
+          ),
+        ],
+      ),
+    ),
+    if (!kIsWeb)
+      TypeSelector(
+        type: FileType.image,
+        selector: Row(
+          children: <Widget>[
+            Icon(Icons.add_photo_alternate),
+            Padding(
+              padding: const EdgeInsets.only(left: 8.0),
+              child: Text("Add images"),
+            ),
+          ],
+        ),
+      ),
+    ],
+  )
 ```