Improve documentation for Docker systemd images

Add detailed usage instructions and examples for running Docker images with systemd, and a section on contributing to the repository in `README.md`.

* **Usage Instructions**
  - Add instructions for running Docker images with `systemd`.
  - Provide command example for running Docker images with `systemd`.

* **Examples**
  - Add example for running Ubuntu 20.04 Docker image with `systemd`.
  - Add example for running Ubuntu 22.04 Docker image with `systemd`.
  - Add example for running Ubuntu 24.04 Docker image with `systemd`.

* **Contributing**
  - Add a section on contributing to the repository.
  - Reference the Contributor Covenant Code of Conduct.
  - Reference the CODEOWNERS file.
  - Provide steps for forking the repository, creating a new branch, and submitting a pull request.

---

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README.md

@@ -14,6 +14,45 @@ Docker images that runs `systemd` with a minimal set of services.
 
 ## Usage ##
 
+### Running Docker images with systemd ###
+
+To run the Docker images with `systemd`, use the following command:
+
+```sh
+docker run --privileged --volume /sys/fs/cgroup:/sys/fs/cgroup:ro -d <image_name>
+```
+
+Replace `<image_name>` with the appropriate image tag (e.g., `docker-systemd:ubuntu-20.04`).
+
+### Examples ###
+
+#### Running Ubuntu 20.04 Docker image with systemd ####
+
+```sh
+docker run --privileged --volume /sys/fs/cgroup:/sys/fs/cgroup:ro -d docker-systemd:ubuntu-20.04
+```
+
+#### Running Ubuntu 22.04 Docker image with systemd ####
+
+```sh
+docker run --privileged --volume /sys/fs/cgroup:/sys/fs/cgroup:ro -d docker-systemd:ubuntu-22.04
+```
+
+#### Running Ubuntu 24.04 Docker image with systemd ####
+
+```sh
+docker run --privileged --volume /sys/fs/cgroup:/sys/fs/cgroup:ro -d docker-systemd:ubuntu-24.04
+```
+
+## Contributing ##
+
+We welcome contributions to this repository. Please follow the guidelines below:
+
+1. Read and follow the [Contributor Covenant Code of Conduct](.github/CODE_OF_CONDUCT.md).
+2. Review the [CODEOWNERS](.github/CODEOWNERS) file to understand the code ownership and review process.
+3. Fork the repository and create a new branch for your feature or bugfix.
+4. Submit a pull request with a clear description of your changes.
+
 ## License ##
 
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)