[Docs] Update sparse-fieldsets chapter (#462)

#193

Author: myfbone

docs/fetching/sparse-fieldsets.md

@@ -1,3 +1,9 @@
 # Sparse Fieldsets
 
-@todo
+## Introduction
+
+This package supports the [sparse fieldsets](https://jsonapi.org/format/#fetching-sparse-fieldsets). 
+This allows a client to specify optional fields for resources.
+You can find additional information in json api [specification](https://jsonapi.org/format/#fetching-sparse-fieldsets) and at this [issue on github](https://github.com/cloudcreativity/laravel-json-api/issues/193).
+
+```This docs page will be updated```