Add README file

Add a README file which contains some information about the
repository. Short installation instructions are included as
well.
Show depth values

* Increase the zoom level for the base map.
* Configure the base map to zoom the tile images on client side
  for zoom levels > 19.
* Change layer names for the track point layer.

[dfa@bezono.org: changed commit message.]
[dfa@bezono.org: removed superfluous method 'createLayer' (dead code).]
[dfa@bezono.org: did some minor code style changes.]
Signed-off-by: Dominik Fässler <dfa@bezono.org>