Document the workflow of updating the map and list of installations

[pdurbin]

This comment from #42 (comment) indicates that the workflow should be documented.

"I took a quick look at the docs and I could probably figure it out but I want to better understand the workflow first. Does all of this process just serve to update the https://github.com/IQSS/dataverse-installations/blob/master/data/data.json file which I assume powers the map? If that's the case, why not just make PRs directly against that file?"

The README is probably a good enough place for this information.