Added URL in README.md.

doc.zih.tu-dresden.de/README.md

@@ -247,7 +247,8 @@ There are two important branches in this repository:
 - Preview:
   - Branch containing recent changes which will be soon merged to main branch (protected
     branch)
-  - Served at [todo url](todo url) from TUD VPN
+  - Served at [https://doc.zih.tu-dresden.de/preview](https://doc.zih.tu-dresden.de/preview) from
+    TUD VPN
 - Main: Branch which is deployed at [https://doc.zih.tu-dresden.de](https://doc.zih.tu-dresden.de)
     holding the current documentation (protected branch)