Update license to `BSD-3-Clause OR Apache-2.0`
All past contributors have agreed to additionally offering the Apache 2.0 license for µD3TN. Contributions submitted for µD3TN must agree to both licenses from now on.
This adds a SPDX-compatible license file plus the corresponding section
in README.md. The file documenting external licenses is moved to external/
.
Edited by Felix Walter