add HIFIS Transfer Service

documentation.md

@@ -16,6 +16,7 @@ Some frequently used documentation links are listed below:
 * [Helmholtz AAI Concepts](https://aai.helmholtz.de/concept)
 * Guidelines for [Managers of Virtual Organisations](https://hifis.net/doc/backbone-aai/guidelines-vos/)
 * Guidelines for [Service Providers](https://hifis.net/doc/backbone-aai/guidelines-services/)
+* How to [use HIFIS Transfer Service](https://hifis.net/doc/core-services/fts-endpoint/)
 
 #### Status Quo of Implementation