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

### Clear and concise description of the problem

feature https://github.com/microsoft/typespec/issues/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

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.