All ClientGen should provide the document to describe what user should do next after generate the code

### Clear and concise description of the problem

In vscode, do the following with TypeSpec extension:

1. invoke the command of TypeSpec: emit from TypeSpec
2. select .NET, Python, Java, JS client code emitter.
3. run the emitter.

Check the output of emitter, there is no readme document to describe what the next step is for user.
Python SDK contains the readme.md, but it is empty.

We shall at least provide some guide to let user start to play with the generated code including how to build, how to write a sample project with sample code, run the sample code targeting the service.

### 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.