Add contribution guideline and pull request template for adding packages

[torik42]

I myself had to look into old pull requests to see what needs to be done. I had the yalafi/packages/package_name.py before. But also other things need to be done. And still, I missed some.

It would be super nice, to have a short list on what needs to be done and also a checkmark list in some pull request template. For example

Add support for LaTeX package .

Checklist

• Add a file yalafi/packages/package_name.py implementing all the needed commands of the package.
• Add a file yalafi/tests/test_packages/test_package_name.py testing all the features.
• Add the package and implemented features to list-of-macros.md and reference it in the Contents section.
• Add the package to the default package list in yalafi/packages/__init__.py.

Hopefully, I did not forget a step. The same, of course, would be nice for document classes.

[matze-dd]

Thank you for this useful feedback and equally useful proposal! I'll definitely come back to this issue.