Add a description on how to control the map

[FlorianK13]

Some users seem to have difficulties controlling the three.js map, especially with a laptop. One user told us that he did not know that he can rotate the map using the right mouse button. Maybe we need some sort of controller legend?

[FlorianK13]

First possibility:

We have some text "Bedienung der Karte" or some icon on the map, maybe upper right corner. If you hover or click on it, you get a popup that explains the buttons and mouse movement.

[FlorianK13]

Second Possibility:

We have a small popup somewhere not too prominent that explains the movement. This popup could dissapear automatically after some time.

[FlorianK13]

@khdlr @kpoeppel @MartGro what do you think? Any other suggestions?

[khdlr]

I like the 1st option, we could just add another button next to the existing overlay buttons