op-service: add readme (#13162)

op-service/README.md

+# `op-service`
+
+Issues: [monorepo](https://github.com/ethereum-optimism/optimism/issues?q=is%3Aissue%20state%3Aopen%20label%3AA-op-service)
+
+Pull requests: [monorepo](https://github.com/ethereum-optimism/optimism/pulls?q=is%3Aopen+is%3Apr+label%3AA-op-service)
+
+`op-service` is a collection of Go utilities to build OP-Stack services with.
+
+```text
+├── cliapp          - Flag and lifecycle handling for a Urfave v2 CLI app.
+├── client          - RPC and HTTP client utils
+├── clock           - Clock interface, system clock, tickers, mock/test time utils
+├── crypto          - Cryptography utils, complements geth crypto package
+├── ctxinterrupt    - Blocking/Interrupt handling
+├── dial            - Dialing util functions for RPC clients
+├── endpoint        - Abstracts away type of RPC endpoint
+├── enum            - Utils to create enums
+├── errutil         - Utils to work with customized errors
+├── eth             - Common Ethereum data types and OP-Stack extension types
+├── flags           - Utils and flag types for CLI usage
+├── httputil        - Utils to create enhanced HTTP Server
+├── ioutil          - File utils, including atomic files and compression
+├── jsonutil        - JSON encoding/decoding utils
+├── locks           - Lock utils, like read-write wrapped types
+├── log             - Logging CLI and middleware utils
+├── metrics         - Metrics types, metering abstractions, server utils
+├── oppprof         - P-Prof CLI types and server setup
+├── predeploys      - OP-Stack predeploy definitions
+├── queue           - Generic queue implementation
+├── retry           - Function retry utils
+├── rpc             - RPC server utils
+├── safego          - Utils to make Go memory more safe
+├── serialize       - Binary serialization abstractions
+├── signer          - CLI flags and bindings to work with a remote signer
+├── solabi          - Utils to encode/decode Solidity ABI formatted data
+├── sources         - RPC client bindings
+├── tasks           - Err-group with panic handling
+├── testlog         - Test logger and log-capture utils for testing
+├── testutils       - Simplified Ethereum types, mock RPC bindings, utils for testing.
+├── tls             - CLI flags and utils to work with TLS connections
+├── txmgr           - Transaction manager: automated nonce, fee and confirmation handling.
+└── *.go            - Miscellaneous utils (soon to be deprecated / moved)
+```
+
+## Usage
+
+From `op-service` dir:
+```bash
+# Run Go tests
+make test
+# Run Go fuzz tests
+make fuzz
+```
+
+## Product
+
+### Optimization target
+
+Provide solid reusable building blocks for all OP-Stack Go services.
+
+### Vision
+
+- Remove unused utilities: `op-service` itself needs to stay maintainable.
+- Make all Go services consistent: `op-service` modules can be used to simplify and improve more Go services.
+
+## Design principles
+
+- Reduce boilerplate in Go services: provide service building utils ranging from CLI to testing.
+- Protect devs from sharp edges in the Go std-lib: think of providing missing composition,
+  proper resource-closing, well set up network-binding, safe concurrency utils.
+
+## Testing
+
+Each op-service package has its own unit-testing.
+More advanced utils, such as the transaction manager, are covered in `op-e2e` as well.