chore: README with basic summary

README.md

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+# Optics
+
+OPTimistic Interchain Communication
+
+## Abstract
+
+We present Optics - a system for sending messages between consensus systems
+without paying header validation costs by creating the illusion of cross-chain
+communication. Similar to an atomic swap, Optics uses non-global protocol
+validation to simulate cross-chain communication. Optics can carry arbitrary
+messages (raw byte vectors), uses a single-producer multi-consumer model, and
+has protocol overhead sublinear in the number of messages being sent.
+
+## Key Points
+
+System sketch:
+
+1. A "home" chain commits messages in a merkle tree
+2. A bonded "updater" attests to the commitment
+3. The home chain ensures the attestation is accurate, and slashes if not
+4. Attested updates are replayed on any number of "replica" chains, after a
+   time delay
+
+As a result, one of the following is always true:
+
+1. All replicas have a valid commitment to messages from the home chain
+2. Failure was published before processing, and the updater can be slashed on
+   the home chain
+
+This guarantee, although weaker than header-chain validation, is still likely
+acceptable for most applications.
+
+## Summary
+
+Optics is a new strategy for simulating cross-chain communication without
+validating headers. The goal is to create a single short piece of state (a
+32-byte hash) that can be updated regularly. This hash represents a merkle tree
+containing a set of cross-chain messages being sent by a single chain (the
+"home" chain for the Optics system). Contracts on the home chain can submit
+messages, which are put into a merkle tree (the "message tree"). The message
+tree's root may be transferred to any number of "replica" chains.
+
+Rather than proving validity of the commitment, we put a delay on message
+receipt, and ensure that failures are publicly visible. This ensure that
+participants in the protocol have a chance to react to failures before the
+failure can harm them. Which is to say, rather than preventing the inclusion of
+bad messages, Optics guarantees that message recipients are aware of the
+inclusion, and have a chance to refuse to process them.
+
+To produce this effect, the home chain designates a single "updater." The
+updater places a bond ensuring her good behavior. She is responsible for
+producing signed attestations of the new message tree root. The home chain
+accepts and validates these attestations. It ensures that they extend a
+previous attestation, and contain a valid new root of the message set. These
+attestations are then sent to each replica.
+
+The replica accepts an update attestation signed by the updater, and puts it in
+a pending state. After a timeout, it accepts the update from that attestation
+and stores a new local root. Because this root contains a commitment of all
+messages sent by the home chain, these messages can be proven (using the
+replica's root) and then dispatched to contracts on the replica chain.
+
+The timeout on new updates to the replica serves two purposes:
+
+1. It ensures that any misbehavior by the updater is published **in advance**
+   of message processing. This guarantees that data necessary for home chain
+   slashing is available for all faults.
+2. It gives message recipients a chance to opt-out of message processing for
+   the update. If an incorrect update is published, recipients always have the
+   information necessary to take defensive measures before any messages can be
+   processed.