Add Data Donation markdown file

[EmilyWes]

This serves as a general description of the criteria to get a dataset in to SYNERGY Plus.
Links to the new form.

[Rensvandeschoot]

To clarify:
Synergy contains the 26 labelled datasets as published here: https://doi.org/10.34894/HE6NAQ

Synergy Plus will contain:

• 24 of the 26 Synergy datasets, see Data quality issue: Chou_2004 #127 and 4ab9382
• improve quality for the initial Synergy datasets, see for example Update Muthu inclusions #158 Add corrected version of Moran #128
• new datasets

With the current Pull Request, we hope the community will contribute more datasets to populate Synergy Plus!!

[EmilyWes]

Thanks @Rensvandeschoot for the review! PR is updated.