Move docs to spec

spec/coordinator/Coordinator.md

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+# Coordinator
+
+The coordinator is a service which communicates with all of the processors,
+all of the other coordinators over a secondary P2P network, and with the Serai
+node.
+
+This document primarily details its flow with regards to the Serai node and
+processor.
+
+### New Set Event
+
+On `validator_sets::pallet::Event::NewSet`, the coordinator spawns a tributary
+for the new set. It additionally sends the processor
+`key_gen::CoordinatorMessage::GenerateKey`.
+
+### Key Generation Event
+
+On `validator_sets::pallet::Event::KeyGen`, the coordinator sends
+`substrate::CoordinatorMessage::ConfirmKeyPair` to the processor.
+
+### Batch
+
+On `substrate::ProcessorMessage::Batch`, the coordinator notes what the on-chain
+`Batch` should be, for verification once published.
+
+### SignedBatch
+
+On `substrate::ProcessorMessage::SignedBatch`, the coordinator publishes an
+unsigned transaction containing the signed batch to the Serai blockchain.
+
+### Sign Completed
+
+On `sign::ProcessorMessage::Completed`, the coordinator makes a tributary
+transaction containing the transaction hash the signing process was supposedly
+completed with.
+
+Due to rushing adversaries, the actual transaction completing the plan may be
+distinct on-chain. These messages solely exist to coordinate the signing
+process, not to determine chain state.