Suggest installing cmdstanr

[jamesmbaazam]

The package now provides some cmdstanr functionalities that can only be accessed if cmdstanr is installed. We should suggest installing it in the README.

[sbfnk]

I think suggesting it might go too far (as people might not be able e.g. to compile code) but I do agree it's a good idea to say that it's an option that gives access to that functionality.

[jamesmbaazam]

I think we are likely saying the same thing. So, we'll mention that {cmdstanr} can be installed to access more models.