Installation instructions for adding TypeSpec to existing projects

Hello! I came across TypeSpec while looking for a way to document my application's API and it looks great. However, the installation instructions only point me toward using `tsp init`, which will overwrite a lot of files in an existing project. It seems like `tsp init` is meant for projects where the API documentation is meant to be a reference for many different teams and thus to stand apart from any actual implementation code.

In my case, I plan to use TypeSpec as a way to document my API and nicely emit an OpenAPI schema. I'd love to see some installation instructions for adding TypeSpec to a project where the .tsp files will sit alongside existing code.

Thanks!