Add link to full MEPS dataset

[joeloskarsson]

Describe your changes

Update the README to include a link for downloading the full MEPS dataset used in existing papers. This is a pure documentation change.

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[leifdenby]

LGTM :) I guess the full MEPS dataset you link to doesn't include a datastore config, right? Not saying we should add that now somewhere (in the README?), but without that it might be hard for people to train a model now we use the datastores to read in data to the model.

[joeloskarsson]

Yes, this was mainly for re-producing the papers with those branches.
But I agree that it would be good to also share such a config. Or at least make a note here how this data can actually be used. What I want to avoid however is people thinking that they can just load this data using a datastore config and run experiments comparable to what we have done in papers. Since the MEPS datastore still has some limitations, that is not the case now, so we should clarify that I think.

I would like write some note about these things before merging this.