added debugging info

EXTRAS/ldap.md

@@ -12,3 +12,6 @@ auth_method: [ldap]
 ldap_servers: ["ldap.in-ulm.de"]
 ldap_base: "cn=users,dc=in-ulm,dc=de"
 ```
+
+### Debugging
+docker run -it --rm alpine /bin/sh -c "apk add openldap-clients;ldapsearch -x -b 'cn=users,dc=in-ulm,dc=de' -H 'ldap://ldap.in-ulm.de'"