Add readme to describe different schema options

deploy/schemas/notifications/README.md

@@ -0,0 +1,17 @@
+# Notifications schemas
+
+There are three schemas presented here, demonstrating different combinations of options to model this problem.
+
+There are two relevant dimensions to decide what schema to use:
+
+- Should we have relationships for subscribers, unsubscribers, or both?
+- Should we use relationships per event type or define event type as a caveat?
+
+|                            | Relationships per subscriber        | Relationships per unsubscriber | Relationships for both unsubscriber & subscriber |
+|----------------------------|-------------------------------------|--------------------------------|--------------------------------------------------|
+| Event type in relationship | [schema_subscribers][1]             | [schema_unsubscribers][3]      | No example yet                                   |
+| Event type in caveat       | [schema_subscribers_event_types][2] | No example yet                 |                                                  |
+
+[1]: ./schema_subscribers.yaml
+[2]: ./schema_subscribers_event_types.yaml
+[3]: ./schema_unsubscribers.yaml