Add Zenodo badge to README

[schillic]

I do not know how to obtain the version-independent badge. @mforets?

[lucaferranti]

10.5281/zenodo.2555934

it's on the right side at the bottom of the "versions" box

[schillic]

Yes, but there is a way to add a badge that auto-updates.
Example: https://github.com/JuliaReach/LazySets.jl/blob/master/README.md

[schillic]

For the record, one adds the following to README.md:

[![zenodo][doi-img]][doi-url]

[doi-img]: https://zenodo.org/badge/XXX.svg
[doi-url]: https://zenodo.org/badge/latestdoi/XXX

where XXX is GitHub's repository ID, which can be found with this algorithm.