add comment about consistent user namespace mapping

etc/systemd/nspawn/xcache.nspawn

@@ -6,6 +6,12 @@ BindReadOnly=/etc/xrootd
 [Exec]
 PrivateUsers=False
 
+# This might be one way to deal with permissions of bind mounts
+# e.g. if the xrootd user inside the container has user id 999 it will consistently
+# become 655360 + 999 on the host system
+#PrivateUsers=655360
+#PrivateUsersOwnership=chown
+
 # Enable this to always make temporary containers (don't write back to the image)
 #Ephemeral=True
 #LinkJournal=False