Enhance CONTRIBUTE.md with commit and PR guides

[joanroig]

In the CONTRIBUTING.md file:

• Add autoclose issue guide: https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue
• Add conventional commits guide, like this (extracted from https://pub.dev/packages/flutter_distributor):

Contributing

If you'd like to contribute to this project, please follow these steps:

1. Fork this repository.
2. Create a new branch for the feature or fix you want to work on.
3. Make your changes and commit them to that branch using the following commit message conventions:
   • feat: For adding new algorithms or features.
   • fix: For modifying existing algorithms or fixing issues.
   • docs: For changing or creating documentation.
   • add (optional): For adding algorithms or features.
4. Submit a pull request to the main repository.