Add standard project files

[ekaf]

The repository is missing several standard files that are important for open-source projects. To improve clarity and encourage contributions, I suggest adding the following files and updates:

1. LICENSE: Clearly state the license for the repository itself. Add a section clarifying that the license for individual datasets may differ (e.g., some datasets such as Punkt have unknown or separate licenses), and document these cases explicitly.
2. CONTRIBUTING.md: Provide guidelines for contributing, including detailed instructions for adding a new dataset. To avoid duplication, the README should reference this section rather than repeating instructions.
3. README enhancements:
   • Reference the CONTRIBUTING.md for instructions on adding datasets, rather than duplicating content.
   • Consider adding a brief note on licensing and the existence of package-specific licenses.
4. (Optional) CODE_OF_CONDUCT.md: Set expectations for community behavior.
5. (Optional) .github/ISSUE_TEMPLATE/: Issue and dataset submission templates to help streamline contributions.

Adding these files and clarifications will help new contributors understand the project and encourage responsible and consistent contributions, while addressing the specific licensing situation of some datasets.

Please note that Copilot wrote the previous paragraphs. Maybe the next step could be to create sub-issues for each of the proposed files?

[ekaf]

Let's close this, since the main files are in place now. Hopefully no need will arise for specifying a code of conduct :)