document the usage of Transform for types in geo-types

[categulario]

Hi folks!

Great work with this bindings and the whole georust ecosystem.

In this PR I propose to add to both the readme and the lib.rs docstring a comment on how to transform structures from geo-types. It took me half an hour and digging into the issues to learn that was possible and how to do it, so not ideal. I would like to save someone the research time so they can focus on using this great project!

✌️

[categulario]

Just as a side note, a friend that uses this project thought it wasn't actually possible to transform entire types of geo-types because he couldn't find it in the docs.

[michaelkirk]

bors r+

Thanks!

[bors[bot]]

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