docs: add django-unfold-markdown to community extensions

[sergei-vasilev-dev]

Add reference to django-unfold-markdown - a community extension providing Markdown editor widget with EasyMDE integration.

This is a separate package that provides Markdown editing capabilities for Django Unfold admin.

Repository: https://github.com/sergei-vasilev-dev/django-unfold-markdown

[lukasvinclav]

Thanks for the package. I added it into README but I chosen different formatting #1713

1. In your repo, the license file is not okay
2. In your PR, you are using "sergei" and "sergey". Which one is correct?

[sergei-vasilev-dev]

Thanks for mentioning! I fixed the license and author name.