editor.py: add a comment including authorship and licence.

Ben is trying to make Bedstead into a Debian package, which involves
clarifying copyright and licences anywhere it's unclear. When I
contributed this editing tool I did it on a very informal basis ('look
at this fun hack!') and didn't make that clear. Time to fix that.

editor.py

@@ -1,5 +1,12 @@
 #!/usr/bin/env python
 
+# Interactive glyph editor for Bedstead.
+#
+# This program was written by Simon Tatham in 2013.
+#
+# Simon Tatham makes this program available under the CC0 Public
+# Domain Dedication.
+
 from __future__ import division, print_function, unicode_literals
 
 import re