* set requirements message

Readme.md

@@ -6,6 +6,15 @@ Introduction
 This module is an Open Source Software of the University Library Leipzig.
 The module retrieve the data of [BSZ One Stop Search (BOSS)](https://wiki.bsz-bw.de/doku.php?id=projekte:boss:start).
 
+Requirements
+------------
+
+Only Records with
+* ISBN/ISBN
+* ZDB ID
+
+will be retrieve the worldcat data.
+
 Installation
 ------------