document intended usage add add reference implementation

[willtemperley]

Just looking at:
https://wiki.openstreetmap.org/wiki/Overpass_turbo/Polygon_Features

I'm wondering why area=no isn't simply blacklisted in polygon-features.json?

[tyrasd]

That's because how this is set up, area!=no is a prerequisite before one can go on to check further tags (just like that a feature must be a closed way to start with). Note that all the cases in polygon-features.json are or-ed together. So we can't simply blacklist area=no there, because then if the object had at least one other area-ish tag (e.g. barrier=wall + area=no), it could falsely be classified as an area.

I admit that there should be at least some documentation about this in the readme, and possibly also a basic reference implementation.