Prepare documentation for external contributions

[unlobito]

To ensure this project is welcoming to contributions from the general public, contribution documentation (e.g. CONTRIBUTING.md, PULL_REQUEST_TEMPLATE.md, ISSUE_TEMPLATE.md) should be provided.

Background

GitHub Documentation

• Configuring issue templates for your repository
• Creating a pull request template for your repository
• Setting guidelines for repository contributors

Completion criteria

• [ ] Contribution guidelines are available at CONTRIBUTING.md
• [x] Code of Conduct is available at CODE_OF_CONDUCT.md (#9)
• [ ] Templates for bugs, feature requests, and tasks are configured in the .github/ISSUE_TEMPLATE folder
• [ ] Template for Pull Requests is available at .github/pull_request_template.md