Update README.md

Author: SzymonFilipek

README.md

@@ -25,6 +25,7 @@ Like what we do? Want to join us? Check out our job listings on our [career page
 ***
 
 * [Overview](#overview)
+* [Functionalities](#functionalities)
 * [Installation](#installation)
 * [Usage](#usage)
 * [About us](#about-us)
@@ -45,6 +46,10 @@ The implementation limits to writing a shipping provider gateway configuration f
 
 If you are curious about the details of this plugin, read [this blog post](https://bitbag.shop/blog/bitbag-shipping-export-plugin-simple-way-to-control-shipments-in-your-online-store).
 
+## Functionalities
+
+If you are curious about the details of this plugin and its finctionalities, check [this file](https://github.com/BitBagCommerce/SyliusShippingExportPlugin/blob/master/doc/functionalities.md).
+
 ## Installation
 
 ```bash