Provide more comprehensive parsing examples

[urschrei]

Having just spent a week working with this library, I think we need some more comprehensive examples showing how to use it, and to call out some corner cases. GeoJSON has a relatively simple spec, but there are subtleties that make it tricky to use:

• FeatureCollections are not straightforward to match against if you want to extract Feature geometries, because they're Option
• GeometryCollections and the fact that they can nest. Not a big deal in practice because this is discouraged and they're not in wide use anyway, but you have to deal with the possibility
• Constructing a Feature with properties set to None results in invalid GeoJSON. We should handle this more ergonomically.
• The try_into trait is available, but not visible in the docs.

To that end, here's #85