Adding license information for demo files

The demo files are covered by the CC-BY-SA-4.0 license.
Please note some discussion about this on the developers mailing list:

https://lists.launchpad.net/kicad-developers/msg39174.html
(cherry picked from commit 444050c7d5)

LICENSE.README

@@ -1,19 +1,21 @@
-The majority of KiCad's source code is developed and distributed under the terms
-of the GPLv3 or later. However, It does include some third-party code licensed
-under AGPLv3 or later as well as sections licensed under the BOOST license v1.0
-and portions licensed under the ISC license.
-
-These licenses are compatible, but a combined works as is will be governed under
-the terms of the AGPLv3 (or later). This includes any binary distribution of the
-KiCad EDA suite by the KiCad project or any third party, e.g. Linux distributor.
-
-You are free to use the *sources* under the terms of their respective licenses.
-
-Licensed under AGPLv3 (or later):
-- TTL [https://www.sintef.no/projectweb/geometry-toolkits/ttl/], sources in include/ttl/*
-Licensed under BOOSTv1:
-- libcontext [https://github.com/boostorg/context], sources in common/system/libcontext.cpp
-Licensed under ISC:
-- portions of code in include/geometry/polygon_triangulation.h
-Licensed under GPLv3 (or later):
-- All remaining code not listed above
+The majority of KiCad's source code is developed and distributed under the terms
+of the GPLv3 or later. However, It does include some third-party code licensed
+under AGPLv3 or later as well as sections licensed under the BOOST license v1.0
+and portions licensed under the ISC license.
+
+These licenses are compatible, but a combined works as is will be governed under
+the terms of the AGPLv3 (or later). This includes any binary distribution of the
+KiCad EDA suite by the KiCad project or any third party, e.g. Linux distributor.
+
+You are free to use the *sources* under the terms of their respective licenses.
+
+Licensed under AGPLv3 (or later):
+- TTL [https://www.sintef.no/projectweb/geometry-toolkits/ttl/], sources in include/ttl/*
+Licensed under BOOSTv1:
+- libcontext [https://github.com/boostorg/context], sources in common/system/libcontext.cpp
+Licensed under ISC:
+- portions of code in include/geometry/polygon_triangulation.h
+Licensed under CC-BY-SA-4.0:
+- All the demo files provided in demos/*
+Licensed under GPLv3 (or later):
+- All remaining code not listed above