update README

vimtabdiff.py

@@ -1,5 +1,9 @@
 #!/usr/bin/python3
 
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
 import os
 import argparse
 import itertools