add readme for bridge indexer (#19912)

## Description 

as title

## Test plan 

How did you test the new or updated feature?

---

## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

For each box you select, include information after the relevant heading
that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:
- [ ] REST API:

---------

Co-authored-by: ronny-mysten <[email protected]>

crates/sui-bridge-indexer/README.md

@@ -0,0 +1,41 @@
+## Overview
+
+Sui Bridge Indexer is a binary that scans Sui Bridge transactions on Sui and Ethereum networks, and indexes the processed data for further use.
+
+## Get Binary
+
+```bash
+cargo build --bin bridge-indexer --release
+```
+
+The pre-built Docker image for Bridge Indexer can be found in `mysten/sui-tools:{SHA}`
+
+## Run Binary
+
+```
+bridge-indexer --config-path config.yaml
+```
+
+
+## Config
+
+```yaml
+---
+remote_store_url: https://checkpoints.mainnet.sui.io
+eth_rpc_url: {eth rpc url}
+sui_rpc_url: {sui rpc url}
+
+concurrency: 500
+checkpoints_path: {path-for-checkpoints}
+
+eth_sui_bridge_contract_address: 0xda3bD1fE1973470312db04551B65f401Bc8a92fD # <-- mainnet, 0xAE68F87938439afEEDd6552B0E83D2CbC2473623 for testnet
+metric_port: {port to export metrics}
+
+sui_bridge_genesis_checkpoint: 55455583 # <-- mainnet, 43917829 for testnet
+# genesis block number for eth
+eth_bridge_genesis_block: 20811249 # <-- mainnet, 5997013 for testnet
+
+eth_ws_url: {eth websocket url}
+
+```
+