Add example for writing in examples/

[michaelkirk]

Someone in the georust discord was asking for help writing a shapefile. I don't use this crate much, but I wanted to help point them in the right direction.

There are API docs at https://docs.rs/shapefile/latest/shapefile/writer/struct.Writer.html but an end-to-end "writing" example, like you have for reading, would be helpful.

[tmontaigu]

Yes that is a good idea, I'll try to add one