Add CONTRIBUTING.md documentation

[tphakala]

I should write contributing instructions, here are at least some topics which should be covered

Topics to Include in CONTRIBUTING.md

1. Introduction

• Brief overview of the project.
• The importance of contributions and how they can help.
• Encourage contributions of all kinds (code, documentation, ideas, etc.).

2. Code of Conduct

• Link to the project's Code of Conduct.
• Emphasize the importance of respectful and inclusive interactions.

3. Getting Started

• Instructions on setting up a development environment, preferred OS etc.
• Steps to install the Go compiler and any other prerequisites.

4. How to Contribute

• Steps to fork the repository and clone it locally.
• How to set up the development branch and keep it in sync with the main project.
• Guidelines for creating a new feature or fixing a bug (branch naming, coding standards, etc.).

5. Submitting Changes

• Instructions on how to submit a pull request.
• The process project uses to review and merge pull requests.
• What information to include in a pull request (e.g., descriptions of changes, screenshots for UI changes, etc.).

6. Coding Guidelines

• Coding standards specific to Go and project's style (formatting, variable naming, etc.).
• Any linting or formatting tools contributors should use.

7. Testing

• How to write tests for the new features or bug fixes.
• Running existing tests to ensure contributions don't break anything.

8. Documentation

• Guidelines for updating or adding documentation.
• Importance of comments in code for complex logic.

9. Community and Communication

• How contributors can ask for help or discuss features (e.g., GitHub issues, Discord, mailing list, etc.).
• Encourage updates or discussions on ongoing work to avoid duplication of effort.

10. Acknowledgment of Contributors

• How the project recognizes contributors (e.g., listing in the README, a contributors file, etc.).

11. Where to Find More Help

• Additional resources or documentation.
• External links that might be helpful for understanding Go, Linux development environment setup, etc.

[tphakala]

Also

• Internal structure of BirdNET-Go should be explained, how to add new actions, improve Web interface etc.
• Describe preferred libraries and tooling for Web UI