add COPYING file

We use the simple licence notice text provided here
https://www.gnu.org/prep/maintain/html_node/License-Notices-for-Other-Files.html

It was already in git-latexdiff's header, but here's a more prohiminent
and easy to find notice.

Fixes #9.

COPYING

@@ -0,0 +1,6 @@
+Main author: Matthieu Moy <Matthieu.Moy@imag.fr> (2012 - 2016)
+
+Copying and distribution of git-latexdiff and its testsuite, with or
+without modification, are permitted in any medium without royalty
+provided the copyright notice and this notice are preserved. This file
+is offered as-is, without any warranty.

git-latexdiff

@@ -1,6 +1,6 @@
 #! /bin/bash
 
-# Main author: Matthieu Moy <Matthieu.Moy@imag.fr> (2012 - 2015)
+# Main author: Matthieu Moy <Matthieu.Moy@imag.fr> (2012 - 2016)
 # (See the Git history for other contributors)
 #
 # Copying and distribution of this file, with or without modification,