Add readme.

docs/broker.md

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+# Neon broker
+
+Neon storage broker targets two issues:
+- Allowing safekeepers and pageservers learn which nodes also hold their
+  timelines, and timeline statuses there.
+- Avoiding O(n^2) connections between storage nodes while doing so.
+
+This is used
+- By pageservers to determine the most advanced and alive safekeeper to pull WAL from.
+- By safekeepers to synchronize on the timeline: advance
+  `remote_consistent_lsn`, `backup_lsn`, choose who offloads WAL to s3.
+
+Technically, it is a simple stateless pub-sub message broker based on tonic
+(grpc) making multiplexing easy. Since it is stateless, fault tolerance can be
+provided by k8s; there is no built in replication support, though it is not hard
+to add.
+
+Currently, the only message is `SafekeeperTimelineInfo`. Each safekeeper, for
+each active timeline, once in a while pushes timeline status to the broker.
+Other nodes subscribe and receive this info, using it per above.
+
+grpcurl can be used to check which values are currently being pushed:
+```
+grpcurl -proto broker/proto/broker.proto -d '{"all":{}}' -plaintext localhost:50051 neon_broker.NeonBroker/SubscribeSafekeeperInfo
+```

docs/sourcetree.md

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 Below you will find a brief overview of each subdir in the source tree in alphabetical order.
 
+`broker`:
+
+Neon storage broker, providing messaging between safekeepers and pageservers.
+[broker.md](./broker.md)
+
 `/control_plane`:
 
 Local control plane.