Add documentation for Drivers

Warning, you must merge #287 before this pr.
This pr contains the documentation of:
*  `ST_AsKml`
* `ST_AsGeoJson`
* `DBFRead`
* `DBFWrite`
* `GPXRead`
* `GeoJsonRead`
* `GeoJsonWrite`
* `KMLWrite`
* `SHPRead`
* `SHPWrite`
* `file_table`

see #288
This is supposed to be a `LIMIT 1` I think. But you have 3 points.