Document how to add due dates

[githorse]

Do you want to request a feature or report a bug?

feature

What is the current behavior?

It sounds like it's possible to handle due dates in some way outside the core format, but I don't understand how. Could the docs be updated to mention that? Surely many people would like to know how to use that feature.

See related issue on urgency.

[inkarkat]

The schedule add-on is listed on the add-on directory Wiki page. As long as it's not part of todo.txt-cli itself, it would be confusing to mention this more prominently.

Some users are content with the basic out-of-the-box functionality, other more aspiring users will surely find that directory (it's frequently linked to in the discussions or earlier on the now defunct mailing list) and go through it.