Add implementations to Readme.md according to the documentation

[wxh06]

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

- [Java](https://github.com/socketio/socket.io-client-java)
- [C++](https://github.com/socketio/socket.io-client-cpp)
- [Swift](https://github.com/socketio/socket.io-client-swift)
- [Dart](https://github.com/rikulo/socket.io-client-dart)

New behaviour

- [Java](https://github.com/socketio/socket.io-client-java)
- [C++](https://github.com/socketio/socket.io-client-cpp)
- [Swift](https://github.com/socketio/socket.io-client-swift)
- [Dart](https://github.com/rikulo/socket.io-client-dart)
- [Python](https://github.com/miguelgrinberg/python-socketio)
- [.Net](https://github.com/Quobject/SocketIoClientDotNet)

Other information

https://socket.io/docs/#Other-client-implementations

[dasilvacontin]

Thanks, and LGTM! Brings the repo's README.md up-to-date with the list of alternative clients on the documentation/website.

(Thanks @wxh06 for the commit's subject, which made it easier to understand the why behind this change.)

[darrachequesne]

@wxh06 thanks!