Add CLI docs

[mdpiper]

We should add documentation on how to use bmipy-render.

[mcflugen]

Agreed.

Is bmipy-render a dumb name? Should it just be bmipy or bmipy-template? Or bmipy-generate-template, or something else?

[mdpiper]

I think bmipy-render is kinda cool, but if you want to change it, that's OK, too.

[mcflugen]

Sounds good. Let's just keep it as-is, then.