Add missing design document to Makefile.am

...so that it ends up in the release
tar ball as well.
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>

Makefile.am

@@ -674,6 +674,7 @@ docinput = \
 	doc/design-shared-storage.rst \
 	doc/design-ssh-ports.rst \
 	doc/design-storagetypes.rst \
+	doc/design-sync-rate-throttling.rst \
 	doc/design-systemd.rst \
 	doc/design-upgrade.rst \
 	doc/design-virtual-clusters.rst \