Add documentation on how to setup stylesheet without a DEM

[pnorman]

For development purposes, hillshading is not always required.

It would be good to be able to set up the stylesheet with an empty DEM or provide an empty hillshading file

[yohanboniface]

You can add active: false on the layers through localconfig. I'll add a mention and an example for that :)

[yohanboniface]

Added in https://github.com/hotosm/HDM-CartoCSS/wiki/Open-With-Kosmtik#download-and-preprocess-dem