[vector-api] note about showing icons on low map scales

Nice work on the KML example, great dataset.

http://twpayne.github.io/ol3/vector-api/examples/kml.html?q=vector

One thing I noticed is that when you zoom out far, it looks like the icons are not in their correct positions (e.g. the P icon looks like it's in between Netherlands and UK).

Maybe they should not be shown at these levels?

![screen shot 2013-12-20 at 18 53 27](https://f.cloud.github.com/assets/319678/1792460/f013eac8-699f-11e3-98b4-8cc92c356bc1.png)