Make adding markers to the map more intuitive

Currently, markers are added by double-clicking, and we don't explain the functionality at all. In the usability test, the subject couldn't figure out how to add a marker to the map.

We should investigate ways to make explain the feature more clearly or make it somehow more intuitive.

Either:

• Add introduction on how to add markers (Easy, but might clutter the view quickly)
• Find a more intuitive way of adding markers (hard, we would need to investigate all the possible ways and do research)