datalad fmriprep singularity push instruction

[ltetrel]

Do we have documentation on how to push new fmriprep lts images to the datalad repo, including the OSF key @glatard https://github.com/glatard/fmriprep-lts-images/pulls

I could not create an issue in the datalad repo (I think because I have no write perm).