Add CITATION file

[kelle]

Could y'all please add a CITATION file to this repo with the preferred method to cite this package?

We’re updating the Affiliated Package Review guidelines to include this (PR #229) and we’d like all coordinated packages to have one before we merge that change. Thanks!

[lpsinger]

@cdeil, @astrofrog, how would you like people to cite astropy-healpix? I have just been citing the reproject paper. Would you like to create a software DOI for astropy-healpix through Zenodo and leave it at that? Or would you like to do a JOSS submission, or something similar?

[pllim]

Update: I toggled on the Zenodo webhook for this repo, so next time you do a GitHub Release, you will get a DOI. And then you can add that DOI badge and so on.