Add more badges to the readme

[karimbahgat]

Possible suggestions:

• Current version
• Downloads
• Used by
• A list of milestones with progress (via shields.io)

[musicinmybrain]

If you really like badges, there’s https://repology.org/project/pyshp/badges, e.g.:

[karimbahgat]

Thanks for the tip! Being a pure Python package, in theory pyshp should run on any OS that can run Python, so don't think it's necessary with OS support badges. Unless i misunderstood what these badges are for. Another site that provides badges is https://shields.io.