Need documentation clarifying that you can add images / external links in markdown

[mvexel]

Came up in the June monthly community meeting

It is not made explicit that we support loading external images. Even if images by URL are defined in markdown we should make it clear that you can use this in instructions.

[joostschouppe]

And maybe also add that it is NOT possible to add an image to the pop-up for an individual task, like I tried here: https://maproulette.org/challenge/36827/task/143232384