Make the readme more useful

This also adds a minified ol.js file to the distribution, which is
referenced in the readme.
 To get started, `recursively clone` the [OL3-Cesium repository](https://github.com/openlayers/ol3-cesium), and call make dist:
```shell
git clone --recursive https://github.com/openlayers/ol3-cesium/
make dist
```