Update README Please

[wilkystorm]

Hello, I really appreciate this tool. Since the python script was converted to shell script with setup it no longer works for me or I do not understand how to use it. I reverted to older version and was able to use it with the script. When you have time can you please update the README to explain how to use this with the new shell script. Thank you!

[StefanBrand]

You can install the script via

pip install git+https://github.com/nlohmann/swagger_to_uml

This puts the swagger_to_uml on your PATH and you can use it directly on the command line, i.e.

swagger_to_uml openapi.json > openapi.puml