[gnome-backgrounds] add license info for the default wallpapers

[Jakub Steiner <jimmac src gnome org>]

commit b9236568be9ffec6d379911b8338d6b3a9fae975
Author: Jakub Steiner <jimmac gmail com>
Date:   Mon Sep 1 12:16:12 2014 +0200

    add license info for the default wallpapers

 AUTHORS         |    1 +
 COPYING_CCBYSA3 |    7 +++++++
 2 files changed, 8 insertions(+), 0 deletions(-)
---
diff --git a/AUTHORS b/AUTHORS
index 425f710..500c145 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -1,5 +1,6 @@
 Image Authors:
 
+- adwaita*:
 - Chmiri.jpg:
 - Terraform-green.jpg:
   Copyright © 2011 Jakub Steiner <jimmac gmail com>
diff --git a/COPYING_CCBYSA3 b/COPYING_CCBYSA3
new file mode 100644
index 0000000..fc45d78
--- /dev/null
+++ b/COPYING_CCBYSA3
@@ -0,0 +1,7 @@
+This work is licenced under the Creative Commons Attribution-Share Alike 3.0
+United States License. To view a copy of this licence, visit
+http://creativecommons.org/licenses/by-sa/3.0/ or send a letter to Creative
+Commons, 171 Second Street, Suite 300, San Francisco, California 94105, USA.
+
+When attributing the artwork, using "GNOME Project" is enough. 
+Please link to http://www.gnome.org where available.