Decide on an open source license

[niho]

Since this library ha been made publicly available we need to add a proper open source license to avoid liabilities and future troubles. A simple MIT or BSD license (BSD 3 Clause) might probably be best, but we need to have a clear decision on what license to use going forward for our open source code. There are quite a lot of different licenses to choose from (see the SPDX License List).