Add a README.md file

Add a readme that describes the project. Specifically

- module structure
- what we prove and do not prove
- how we test

README.md

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+XMR to BTC Atomic Swap
+======================
+
+This repository is a proof of concept for atomically swapping XMR for BTC.
+
+We define:
+
+- Alice to be the actor that initially holds XMR.
+- Bob to be the actor that initially holds BTC.
+
+The repository is structured as a library and a single test function that executes the swap.
+The library has the following modules:
+
+- `alice`: Defines the state machine that describes the swap for Alice.
+This includes the messages sent to/from Alice.
+- `bob`: Defines the state machine that describes the swap for Bob.
+This includes the messages sent to/from Bob.
+- `bitcoin`: Keys, signing functions, transactions etc. for Bitcoin.
+Also includes a test wallet (see below).
+- `monero`: Keys, signing functions, transactions etc. for Monero.
+Also includes a test wallet (see below).
+
+Currently we have a single test function that proves the following:
+
+- Interaction with both block chains and their respective wallets works.
+- The messages required are correct and can manually drive the state transitions to execute a swap.
+  
+
+Currently we do not do:
+
+- Actual network communication.
+- Watch the blockchain for transactions (we just assume they have been mined as soon as we broadcast and move onto the next state).
+- Verification that the UI is acceptable.
+Since we do everything in a single test function their is no user interaction, this is unrealistic for a real product.
+  
+
+## Testing
+
+We wrote a few additional libraries to facilitate testing:
+
+### Wallets
+
+- `bitcoin` module contains a test wallet by way of `bitcoind`.
+- `monero`: module contains a test wallet by way of `monero-wallet-rpc`.
+  
+### Blockchain harnesses
+
+We have written two harnesses for interacting with bitcoin and monero.
+
+- [bitcoin-harness](https://github.com/coblox/bitcoin-harness-rs)
+- [monero-harness](https://github.com/comit-network/xmr-btc-swap/tree/master/monero-harness)
+
+These harnesses wrap interaction with `bitcoind` and `monerod`/`monero-wallet-rpc`.
+
+We use [testcontainers-rs](https://github.com/testcontainers/testcontainers-rs) to spin up `bitcoind`, `monerod`, and `monero-wallet-rpc` in docker containers during unit/integration testing.
+