Make the readme more useful

This also adds a minified ol.js file to the distribution, which is
referenced in the readme.
We should talk about the use of local projections and automatic reprojection of vectors:
```
You may use imageries in local projection for the ol3 map. In this case, add corresponding imageries in EPSG:4326 or EPSG:3857 to Cesium (http://cesiumjs.org/Cesium/Build/Documentation/ImageryProvider.html).
Vector layers are automatically reprojected to EPSG:4326 during the synchronization to Cesium.
```