README updates: version numbers and updates to cmdstan about bridgestan

[WardBrian]

1:
For Python, we list 3.9 in the setup.cfg file, and the compile_model function uses some standard library features which are 3.9+. It would be reasonable to relax this to as low as 3.7 with some minor rewrites, if we had the desire.

The .C interface for R is so archaic that I'm not sure what a minimum R version would be. The oldest version of R I am able to install is R version 3.3.2 (2016-10-31) -- "Sincere Pumpkin Patch", and it works for that version, so my guess is >=3.

2:
We can, though I'm not sure how much to say. My assumption with the C example is that anyone who knows they want to bind this from a compiled language will probably know how to do it even better than I did

3:
You're right that the prose doesn't make sense, but I think it would be better to cut the line than to add make stan-update. I don't think we should necessarily recommend using develop versions, which is what make stan-update will do.

[roualdes]

1. I'm happy with Python 3.9+, I'm just suggesting we put this info upfront instead of within each package's source code (Julia => Manifest.toml, Python => setup.cfg, src/R => DESCRIPTION). What do you think?

2. Sounds good. Skip.

3. That's a good point, not to recommend develop versions.

[WardBrian]

Yes, I think it's good to put this on the readme and in the relevant sections in https://roualdes.github.io/bridgestan/install.html