[http-client-csharp] generate README for guide of the usage of generated lib

[ArthurMa1978]

Clear and concise description of the problem

feature #6639

The README targets the user of the lib (not the user of the emitter).

It could be similar as https://github.com/Azure/azure-sdk-for-net/blob/a2632e879c3fdbfa35d22b186e2cb019f3b4cda6/sdk/core/System.ClientModel/README.md#accessing-the-service-response

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.