add checklist instructions to user documentation

[nielsgeo]

from experience with users who are new to deck: I suggest to include short info on how to create checklists here:
https://deck.readthedocs.io/en/latest/Markdown/
or here or both?
https://deck.readthedocs.io/en/latest/User_documentation_en/#using-deck
As far as my colleague and me see it is not obvious from current documentation and the links therein that [ ] creates items
Thanks a lot for creating and continuously improving this great free and open trello alternative!

[juliusknorr]

Good point, the documentation is built from https://github.com/nextcloud/deck/blob/master/docs/Markdown.md

Pull requests are welcome 😉