Add RabbitMQ ADR.

architecture-decisions/0003-synchronization-via-rabbitmq.md

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+# 3. Synchronization via RabbitMQ
+
+Date: Archaeological
+
+## Status
+
+Accepted
+
+## Context
+
+We want Pomegranate to be a separate application from Figgy, but need some way
+for Figgy to tell Pomegranate about new resources so that when something is
+marked Complete in Figgy or taken down that it's reflected in Pomegranate.
+
+## Decision
+
+Figgy will send create/update/delete messages to a fanout RabbitMQ Exchange.
+Pomegranate will register a durable queue which listens to that exchange and
+process messages using [Sneakers](https://github.com/jondot/sneakers).
+
+## Consequences
+
+* If Sneakers workers stop processing, then new documents don't make it in to
+Pomegranate and we have to perform a reindex.
+
+* We have to monitor the status of our queues to make sure things are working.