Added example image to featureToH3Set

[matthiasfeist]

When I used the method featureToH3Set in my code it wasn't quite clear to me that it only adds hexagons with the center inside the polygon.
I therefore added a example screenshot and highlighted the passage in the docs to make this clearer (for people like me who like visual examples 😆)

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Coverage remained the same at 100.0% when pulling f2ec9c2 on matthiasfeist:master into b4f8cc9 on uber:master.

[nrabinowitz]

Looks great, thanks!