Document format x to GML conversion options

[stijnvanhoey]

With the addition of the location query options https://github.com/DOV-Vlaanderen/pydov/pull/129 we support the usage of GML to query on location. However, users will probably have other file formats (shapefile, geojson,...) to use within the location search context.

Without supporting more formats within the pydov package, we could point the user to some other references or code examples on how to deal with other file formats.

@jorisvandenbossche, taking into account your experiences on spatial matter, do you have pointers or ideas that we can include in the documentation on how to convert xxxx to GML? with xxxx equal to shapefile, geojson,... (feature file types)