po: README.md: add a note about which Zanata client is required

Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

po/README.md

@@ -4,6 +4,9 @@ Libvirt Message Translation
 Libvirt translatable messages are maintained using the GNU Gettext tools and
 file formats, in combination with the Zanata web service.
 
+python-zanata-client is required in order to use make to pull/push translations
+from/to Zanata server.
+
 Source repository
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