Initial draft for Submission Guidelines

[PMenges]

Submission Guidelines added to Overview (already opened a PR mostly for visibility)

Open questions:

• Does it align with the metadata model or do we modify anything (Karo?)
• Should we expand the "GHGA's Commitment" a bit or reword generally so it fits better into the Overview? (Uli?)
• Should it stay there or should we make a dedicated page for it? (all)

I'll reformat it a bit in the meantime.