doc: license: Add license header to the README.dfutftp file

Signed-off-by: Lukasz Majewski <lukma@denx.de>
diff --git a/doc/README.dfutftp b/doc/README.dfutftp
index 0257f0d..66901e1 100644
--- a/doc/README.dfutftp
+++ b/doc/README.dfutftp
@@ -1,3 +1,11 @@
+#
+#  Copyright (C) 2015
+#
+#  Lukasz Majewski <l.majewski@majess.pl>
+#
+#
+# SPDX-License-Identifier:	GPL-2.0+
+
 Device Firmware Upgrade (DFU) - extension to use TFTP
 =====================================================