Collect OSS libraries notices

[rock3r]

We are using a few dependencies and have borrowed some code (see #90) from externals; we should really be collecting such licensing information in a file (a markdown file on the repo, or something like that) to comply with those libraries' licenses.

[rock3r]

I have set up a redirect from https://stalla.dev/oss-licenses to the not-existing-yet https://github.com/mpgirro/stalla/blob/master/OSS.md

When we get to this we should use that URL as our official OSS licenses URL (I suspect it should be somehow bundled in the JAR and/or in the lib metadata). Using https://stalla.dev/oss-licenses allows us to redirect somewhere else in the future without breaking older versions' links.