Improve Documentation for Unity Orb

### Issue: Improve Documentation for Unity Orb

#### Description

The current documentation for the Unity Orb could be improved to make it easier for users to contribute and test their own changes. Specifically:

1. **Add a link to the `CONTRIBUTING.md` file** in the "How to Contribute" section of the README. This will provide users with a clear path to understand the guidelines and expectations for contributions.

2. **Document how to use a fork or a specific commit of the orb** in workflows. This will help contributors test their changes before submitting a pull request.

---

#### Proposed Solution

To address these points:

1. In the README under the "How to Contribute" section, add:
> For detailed guidelines on contributing, see [[CONTRIBUTING.md](https://chatgpt.com/c/CONTRIBUTING.md)](./CONTRIBUTING.md).

2. Add a new section to the README titled `Development` or `Testing Your Fork`. It could include instructions similar to the following:

---

## Development

If you would like to contribute to this orb and test your changes:

1. **Fork this repository.**
2. **Make your changes** in a new branch.
3. **Pack and publish your fork** to a personal namespace:
```bash
circleci orb pack src > unity-orb.yml
circleci orb publish unity-orb.yml /@dev:first
```
4. **Use your fork in your CircleCI workflow.**
Update your `.circleci/config.yml` to reference your development version:
```yaml
orbs:
unity: /@dev:first
```

Update the repository README, including its "How to Contribute" section, and add the requested development or fork-testing instructions. Reference CONTRIBUTING.md, document the CircleCI orb pack and publish commands, and show the workflow orb reference; the work is done when contributors can find the guidelines and follow the fork-testing process.

Written by the indexing model from the issue text.