add doi to readme

[knaaptime]

we already have a zenodo doi for the package, but its not listed in the readme

[martinfleis]

Make sure to use the one for the repo not for the individual versions.

[knaaptime]

i always manage to screw that up... any tips on how to make sure i get the right one?

[knaaptime]

scratch that. the DOI badge is there,

we just dont also have a bibtex listing in the readme like we do for most other packages

[martinfleis]

That seems to be the wrong one :D. This one should be version agnostic DOI 10.5281/zenodo.3386576.