Add paper.md file

[Robinlovelace]

From ropensci/software-review#420

* [x]  Do you wish to automatically submit to the [Journal of Open Source Software](http://joss.theoj.org/)? If so:

JOSS Options

• The package has an obvious research application according to JOSS's definition.

  • The package contains a paper.md matching JOSS's requirements with a high-level description in the package root or in inst/.
  • The package is deposited in a long-term repository with the DOI:

Any ideas on next steps on this @annakrystalli or @temospena ? Will be great to get the nascent slopes paper published!

[annakrystalli]

Hello @Robinlovelace !

So we've actually stopped co-ordinating with JOSS regarding submissions (we changed the template November 2020 but somehow your submission issue slipped through!). You can still submit of course. Just follow JOSS submission guidelines (which effectively means including a paper.md of your paper and procuring a DOI for the repo). You can mention this review which may expedite the review in JOSS but they will first decide whether the package meets their own criteria for publication.

Sorry for the confusion!