Add license terms to README

README.txt

@@ -1,4 +1,19 @@
-//
-// Documentation for these modules is at http://veins.car2x.org/
-//
+
+Veins - The open source vehicular network simulation framework.
+
+See the Veins website <http://veins.car2x.org/> for a tutorial, documentation,
+and publications.
+
+Veins is composed of many parts. See the version control log for a full list of
+contributors and modifications. Each part is protected by its own, individual
+copyright(s), but can be redistributed and/or modified under an open source
+license. License terms are available at the top of each file. Parts that do not
+explicitly include license text shall be assumed to be governed by the "GNU
+General Public License" as published by the Free Software Foundation -- either
+version 2 of the License, or (at your option) any later version
+(SPDX-License-Identifier: GPL-2.0-or-later). Parts that are not source code and
+do not include license text shall be assumed to allow the Creative Commons
+"Attribution-ShareAlike 4.0 International License" as an additional option
+(SPDX-License-Identifier: GPL-2.0-or-later OR CC-BY-SA-4.0). Full license texts
+are available with the source distribution.