[FR] Provide instructions to add new check box types

[sytone]

Is your feature request related to a problem? Please describe.

I was looking for guidance. I want to add [w] as a new checkbox type that indicates that the task is in a waiting for state.

Describe the solution you'd like

Documentation to add and override the icons shown in the theme or the addition of [w] with a hourglass icon.

Describe alternatives you've considered

Reversing the CSS and adding in the custom CSS but that would mean I would have to do that every time you released a new version to ensure I do not break anything.

Additional context

NA