Add developer documentation to explain room DAG concepts like outliers and state_groups

docs/development/faq.md

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+# Developer FAQ
+
+## What is an `outlier`?
+
+An `outlier` is an arbitrary floating event in the DAG (as opposed to being
+inline with the current DAG). It also means that we don't have the state events
+backfilled on the homeserver and we trust the events *claimed* auth events rather
+than those we calculate and verify to be correct.
+
+
+## What is a `state_group`?
+
+For every non-outlier event we need to know the state at that event. Instead of storing the full state for each event in the DB (i.e. a `event_id -> state` mapping), which is *very* space inefficient when state doesn't change, we instead assign each different set of state a "state group" and then have mappings of `event_id -> state_group` and `state_group -> state`.
+
+