Incoming references / foreign keys

[joepio]

See issue in docs

When you use an external resource, you're creating a dependency on a system that's out of your control. That external system could change the resource, remove it... Anything. In Atomic Data, these links can be very important. Part of dealing with this problem, is using IPFS #66, but that doesn't really work for data that changes over time.

Some usecases:

• Before I delete a Resource, I want to know which external systems depend on this.
• When I delete or edit a Resource I want external systems to be notified of this. Notifications, subscriptions and feeds #77

[joepio]

We now already have the first usecase: see which resources are referencing another one. I'll remove it from the 1.0.0 milestone since it's less urgent to actually do this in the back-end now.