docs: add op-preimage to directory structure

README.md

@@ -57,6 +57,7 @@ Refer to the Directory Structure section below to understand which packages are
 ├── <a href="./op-exporter">op-exporter</a>: Prometheus exporter client
 ├── <a href="./op-heartbeat">op-heartbeat</a>: Heartbeat monitor service
 ├── <a href="./op-node">op-node</a>: rollup consensus-layer client
+├── <a href="./op-preimage">op-preimage</a>: Go bindings for Preimage Oracle
 ├── <a href="./op-program">op-program</a>: Fault proof program
 ├── <a href="./op-proposer">op-proposer</a>: L2-Output Submitter, submits proposals to L1
 ├── <a href="./op-service">op-service</a>: Common codebase utilities