Tweak readme
_The documentation is not available anymore as the PR was closed or merged._ Nit: We should also update the `Disclaimers` section to let the dataset owners know they should use Hub discussions rather than GH issues for removal requests/updates Updated the disclaimers section, thanks !

Does it sound good to you @albertvillanova ?
Tweaked some paragraphs mentioning the modalities we support + added a paragraph on security