Add cause to feature request issue template

Add cause to feature request issue template

When writing https://github.com/openstreetmap/openstreetmap-website/issues/3551 I noticed that your template is somewhat problematic.

What users really should write is how/why they have the feature request they want, i.e. what (problem) is “behind” a feature request. Similar to how user stories should always go like ”I as a [who] want [what], because [why]”, here the “why” aspect was missing.
The “Description” does not really tangle that, it only tangles _what_ the user wants.

Screenshots may also be better inline inside of a text description, but I see you want to nudge the user to add screenshots or mockups, so that's fine IMHO.