JOSS paper submission

[paul-buerkner]

@avehtari, @jgabry, @mjskay this PR adds a short paper (in the paper/ folder) for submission to JOSS as mentioned in #428.

If possible, I would love to submit the paper this week already, so I would kindly asking you do the following in the next few days (it's not much work, I promise):

• Read the paper (file paper.md) and edit it as you see fit. The submission guidelines of JOSS can be found at https://joss.readthedocs.io/en/latest/submitting.html, along with a paper example that I used as orientation when writing the first draft. Likely, there are also some remaining typos so please help me fix them.
• Check your name, affiliation, and ORCID at the top of the paper file.
• I already acknowledge all people listed as contributors in DESCRIPTION in the acknowledgment section. Please check if you feel anyone is missing from that list. In this case, we should add them to both DESCRIPTION and the paper.
• Think about whether any of the contributors should be moved to a full author of the package, since they should then also be listed as author on the paper.
• Add further acknowledgments if applicable.

If you have done all the above, please approve this PR. Once I have received approval from all three of you (or more if more become authors), I will submit this paper to JOSS and also prepare an arxiv preprint.

[avehtari]

Looks good. I made some minor edits and added second affiliation

[jgabry]

This looks good. I fixed some typos and made a few tiny other changes to some of the language, but I didn't change any of the substance. Thanks for putting this together so quickly!

[jgabry]

My affiliation will soon not be Columbia anymore! But technically it still is until the end of this month and I've been at Columbia when I did work on posterior, so I think it's fine to keep it.

[mjskay]

Hi all, sorry for the delay --- I will check this today and get back to y'all.

(Also apologies if I'm behind on any other posterior-related stuff --- a lot of other commitments have really reduced my dev time lately. I am cautiously optimistic I will have more time soon as a number of my students are in the midst of graduating. Personally I am looking forward to having dev time again :)).

[mjskay]

Looks good to me!