Updated DOI with JOSS paper

[jeffjennings]

Following eventual acceptance of the JOSS paper for MPoL, update the citation in README to the @article paper and also request that users cite the @software Zenodo DOI.

Also check that the paper in paper/paper.md is final.

[jeffjennings]

Closing as low priority / not needed

[iancze]

We'll get to the JOSS paper eventually :) But let's strategize about what that looks like after v0.3 is merged.

[jeffjennings]

No rush, just closed as I shouldn't have made this a full issue in the first place. I added a reminder for it in #202