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 and the corresponding section
in README.md as well. The file documenting external licenses is moved to
`external/`.

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