Paper for JOSS

[paquiteau]

This PR is here for contributors to review the paper we would like to submit to the Journal of Open Source Software (JOSS)

Please check the submitting guidelines and update the paper accordingly (notably your affiliation!)

Note that the paper should be 800-1000 words long and that we are a bit longer than that; any proposal to make it shorter is welcomed.

Once we get approval of everybody (do it using the github review), we will release a new version of MRI-NUFFT, and submit it to JOSS.