Creating demo instances

Hello All,

I recently discovered that creating a demo roundup instance may behave 
differently depending on if the user invokes `python demo.py` or 
`roundup-demo`, and this has the potential to cause problems. The 
general summary of the problem (and a bit more context behind starting 
this conversation) can be found at:

   https://github.com/roundup-tracker/roundup/pull/2#issuecomment-786433320

This raises the question, what is the "correct" way to create a demo 
instance?

The `doc/features.txt` mentions running `python demo.py` to get "instant 
gratification" while `dov/installation.txt` mentions both `demo.py` and 
`roundup-demo`.

I'm thinking we should probably standard on a canonical method for 
creating demo instances and update the docs accordingly.

My suggestion would be to standardise on `roundup-demo` as this is 
really the only viable way to create a new demo instance after doing a 
`pip install roundup` (the alternative is to change into the 
`<venv>/lib/python3.X/site-packages/roundup` directory and then run 
`python demo.py`).

Any thoughts?

Cheers,
John.