mkhufftbl.py: Update doc

mkhufftbl.py

@@ -1,4 +1,13 @@
 #!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+# This script reads Huffman Code table [1] and generates symbol table
+# and decoding tables in C language.  The resulting code is used in
+# lib/nghttp2_hd_huffman.h and lib/nghttp2_hd_huffman_data.c
+#
+# [1] http://http2.github.io/http2-spec/compression.html
+
+from __future__ import unicode_literals
 import re
 import sys