For protecting µD3TN from proprietary forks that do not give back to the
open-source project, we decided to re-license the µD3TN project under
the GNU Affero General Public License v3.0 (AGPLv3).

To still enable proprietary projects to use µD3TN, D3TN will start to
offer proprietary licenses. To ensure that all future contributions are
compatible with this dual licensing scheme, all external contributions
will still be accepted under the terms of the Apache 2.0 license.

Signed-off-by: Felix Walter <felix.walter@d3tn.com>