Add README.md ..

Use the template README used across the different Stratum V2 protocol
crates to `common_messages_sv2` crate.

protocols/v2/subprotocols/common-messages/README.md

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+# common_messages_sv2
+
+[![crates.io](https://img.shields.io/crates/v/common_messages_sv2.svg)](https://crates.io/crates/common_messages_sv2)
+[![docs.rs](https://docs.rs/common_messages_sv2/badge.svg)](https://docs.rs/common_messages_sv2)
+[![rustc+](https://img.shields.io/badge/rustc-1.75.0%2B-lightgrey.svg)](https://blog.rust-lang.org/2023/12/28/Rust-1.75.0.html)
+[![license](https://img.shields.io/badge/license-MIT%2FApache--2.0-blue.svg)](https://github.com/stratum-mining/stratum/blob/main/LICENSE.md)
+[![codecov](https://codecov.io/gh/stratum-mining/stratum/branch/main/graph/badge.svg)](https://app.codecov.io/gh/stratum-mining/stratum/tree/main/protocols%2Fv2%2Fcommon_messages_sv2)
+
+`common_messages_sv2` is a Rust crate that implements a set of shared messages used by multiple Stratum V2 roles.
+
+## Usage
+
+To include this crate in your project, run:
+
+```bash
+$ cargo add common_messages_sv2
+```
+
+This crate offers feature flags to enable or disable the use of `std`, `serde` or `quickcheck`
+dependencies.
+