op-fetcher: add README.md (#15280)

Author: bitwiseguy

op-fetcher/README.md

@@ -0,0 +1,21 @@
+## op-fetcher
+
+This is a service that takes `SystemConfigProxy` and `L1StandardBridgeProxy` as inputs, and uses those addresses to fetch additional chain configuration information from onchain. All of the L1 rpc calls are made via a single solidity script, `FetchChainInfo.s.sol`. The compiled `forge-artifacts` for that script are then embedded in the go code so that `op-fetcher` can be used as a go library in addition to a cli tool. Current information fetched from onchain with this service (can be expanded in the future):
+* all L1 contract addresses except `SystemConfigProxy` and `L1StandardBridgeProxy`
+* all L1 roles
+* `fault_proof_status` fields (to determine no fault proofs vs. permissioned vs. permissionless)
+
+## Usage
+1. Build (compiles solidity script used by this service, as well as go code):
+```
+cd <repo-root>/op-fetcher
+just build-all
+```
+2. Fetch data onchain for a single opchain and write results to json file
+```
+./bin/op-fetcher fetch \
+  --l1-rpc-url "<l1-rpc-url>" \
+  --system-config "<system-config-proxy-address>" \
+  --l1-standard-bridge "<l1-standard-bridge-proxy-address>" \
+  --output-file "./.fetcher/out.json"
+```