Make citable

[sverhoeven]

As a reader of the guide I would like to be able to cite this guide using a DOI.

The following steps need to be done

1. Zenodo-GH integration switched on
2. Add .zenodo.json
3. Create GH release
4. Add CITATION.cff
5. Add Zenodo badge to README.md

[jspaaks]

PR #41

[sverhoeven]

Release v0.1.0 has been made see https://zenodo.org/record/3876113 .