Document how and when to add a cluster

[choldgraf]

Summary

From #116 we discovered that there's a "how to add a hub" guide, but these two questions are not answered anywhere:

• "how to add a cluster"
• "when it's appropriate to add a new cluster"

Note these could also be accompanied by a "new cluster" issue template/checklist in pilot-hubs repo.

We likely wish to keep clusters to a minimum but sometimes it's needed. If so, that'll be a good opportunity to draft the guide

Actions

• Write up documentation for each of the questions above

[choldgraf]

This was completed in 2i2c-org/infrastructure#650