Make the readme more useful

This also adds a minified ol.js file to the distribution, which is
referenced in the readme.
We should be clear that we are dealing with a Cesium specific thing, using Cesium API. Giving a link to available terrain providers would make clear there is a choice of providers.

`To use terrain, simply add a terrain provider to the Cesium scene. (http://cesiumjs.org/Cesium/Build/Documentation/TerrainProvider.html)`