More context and details for README

The README was very anonymous up until now. According to the needs expressed in #1 (closed) I have extended it with links to the live deployment of the application, the (software) project blog, the Matrix channel, the Discourse group, Solidarische Landwirtschaft e.V., issues, merge requests, the GitLab project, contributing guidelines and a license.

This further includes a refactor of the Dokku integration and brings a deploy script.

A LICENSE is provided, too.

The contribution guidelines are prepared as a pattern of good practice, but left for collective discussion about what they should contain.