add BDS licence as an alternative

It may help distributors to have a well-established licence in
addition to the short snippet (which doesn't have a name but is
suggested by the FSF for small files :
https://www.gnu.org/prep/maintain/html_node/License-Notices-for-Other-Files.html )

COPYING

@@ -1,6 +1,35 @@
-Main author: Matthieu Moy <Matthieu.Moy@imag.fr> (2012 - 2016)
+Main author: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr> (2012 - 2017)
 
 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.
+
+
+Alternatively, you may distribute git-latexdiff and its testsuite
+under the terms of the BSD licence :
+
+Copyright (c) 2012 - 2017, Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
+All rights reserved.
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright
+  notice, this list of conditions and the following disclaimer.
+* Redistributions in binary form must reproduce the above copyright
+  notice, this list of conditions and the following disclaimer in the
+  documentation and/or other materials provided with the distribution.
+* Neither the name of the University of California, Berkeley nor the
+  names of its contributors may be used to endorse or promote products
+  derived from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND ANY
+EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE REGENTS AND CONTRIBUTORS BE LIABLE FOR ANY
+DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.