[MAID-2652] chore/license: update license to include linking exception

LICENSE

@@ -1,3 +1,24 @@
+                          LINKING EXCEPTION
+
+Routing is distributed under the terms of the GNU General Public
+License (v3) with the following clarification and special exception.
+Linking this library statically or dynamically with other modules is
+making a combined work based on this library. Thus, the terms and
+conditions of the GNU General Public License cover the whole
+combination. As a special exception, the copyright holders of this
+library give you permission to link this library with independent
+modules to produce an executable, regardless of the license terms of
+these independent modules, and to copy and distribute the resulting
+executable under terms of your choice, provided that you also meet,
+for each linked independent module, the terms and conditions of the
+license of that module. An independent module is a module which is not
+derived from or based on this library. If you modify this library, you
+may extend this exception to your version of the library, but you are
+not obliged to do so. If you do not wish to do so, delete this
+exception statement from your version.
+
+----------------------------------------------------------------------
+
                     GNU GENERAL PUBLIC LICENSE
                        Version 3, 29 June 2007
 

README.md

@@ -42,3 +42,9 @@ Example:
 ## License
 
 Licensed under the General Public License (GPL), version 3 ([LICENSE](LICENSE) http://www.gnu.org/licenses/gpl-3.0.en.html).
+
+### Linking exception
+
+Routing is licensed under GPLv3 with linking exception. This means you can link to and use the library from any program, proprietary or open source; paid or gratis. However, if you modify Routing, you must distribute the source to your modified version under the terms of the GPLv3.
+
+See the LICENSE file for more details.