REUSE compliance

[wneessen]

As of today, the go-mail library is officially REUSE-compliant. The idea was inspired by the recent efforts of the cURL project on REUSE compliance. I believe that making things easy for other projects and companies to use a library as well as keeping control for license(s) is essential in the FOSS worlds. Therefore adding the SPDX identifiers and complying with REUSE is the right way.