: License Git Horror Story script under CC0

Someone contacted me about the license of this script.  The code itself is
doubtfully enforcable with US copyright anyway, so let's just put this into
the public domain.

It does have comments, but they're minor.

docs/papers/git-horror-story.txt

@@ -941,6 +941,8 @@ information, delimited by tabs.
 ----
 #!/bin/sh
 #
+# Licensed under the CC0 1.0 Universal license (public domain).
+#
 # Validate signatures on each and every commit within the given range
 ##