fix(xml): add missing CONTRIBUTING link and fix module docstring

Author: markuslf

check-plugins/xml/xml

@@ -6,7 +6,9 @@
 #          https://www.linuxfabrik.ch/
 # License: The Unlicense, see LICENSE file.
 
-"""Have a look at the check's README for further details"""
+# https://github.com/Linuxfabrik/monitoring-plugins/blob/main/CONTRIBUTING.md
+
+"""See the check's README for more details."""
 
 import argparse
 import base64