message: docs(fd): minor readme update
  Minor README update after questions from a third party.
diff: @@ -20,3 +20,47 @@ Once your environment variables have been set, run the service via:
  ```
  yarn start
  ```
  +
  +## Ports
  +
  +- API is exposed at `$FAULT_DETECTOR__HOSTNAME:$FAULT_DETECTOR__PORT/api`
  +- Metrics are exposed at `$FAULT_DETECTOR__HOSTNAME:$FAULT_DETECTOR__PORT/metrics`
  +- `$FAULT_DETECTOR__HOSTNAME` defaults to `0.0.0.0`
  +- `$FAULT_DETECTOR__PORT` defaults to `7300`
  +
  +## What this service does
  +
  +The `fault-detector` detects differences between the transaction results generated by your local Optimism node and the transaction results actually published to Ethereum.
  +Currently, transaction results take the form of [the root of the Optimism state trie](https://medium.com/@eiki1212/ethereum-state-trie-architecture-explained-a30237009d4e).
  +For every Optimism block, the state root of the block is published to the [`StateCommitmentChain`](https://github.com/ethereum-optimism/optimism/blob/39b7262cc3ffd78cd314341b8512b2683c1d9af7/packages/contracts/contracts/L1/rollup/StateCommitmentChain.sol) contract on Ethereum.
  +We can therefore detect differences by, for each block, checking the state root of the given block as reported by an Optimism node and the state root as published to Ethereum.
  +
  +We export a series of Prometheus metrics that you can use to trigger alerting when issues are detected.
  +Check the list of available metrics via `yarn start --help`:
  +
  +```sh
  +> yarn start --help
  +yarn run v1.22.19
  +$ ts-node ./src/service.ts --help
  +Usage: service [options]
  +
  +Options:
  + --l1rpcprovider Provider for interacting with L1 (env: FAULT_DETECTOR__L1_RPC_PROVIDER)
  + --l2rpcprovider Provider for interacting with L2 (env: FAULT_DETECTOR__L2_RPC_PROVIDER)
  + --startbatchindex Batch index to start checking from (env: FAULT_DETECTOR__START_BATCH_INDEX)
  + --loopintervalms Loop interval in milliseconds (env: FAULT_DETECTOR__LOOP_INTERVAL_MS)
  + --port Port for the app server (env: FAULT_DETECTOR__PORT)
  + --hostname Hostname for the app server (env: FAULT_DETECTOR__HOSTNAME)
  + -h, --help display help for command
  +
  +Metrics:
  + highest_checked_batch_index Highest good batch index (type: Gauge)
  + highest_known_batch_index Highest known batch index (type: Gauge)
  + is_currently_mismatched 0 if state is ok, 1 if state is mismatched (type: Gauge)
  + l1_node_connection_failures Number of times L1 node connection has failed (type: Gauge)
  + l2_node_connection_failures Number of times L2 node connection has failed (type: Gauge)
  + metadata Service metadata (type: Gauge)
  + unhandled_errors Unhandled errors (type: Counter)
  +
  +Done in 2.19s.
  +```
  