helpful comments

Added helpful comments to the demo.py
I'd like to avoid putting specific implementations here in case adoption were to grow. Maintaining an address list in one place seems a little bit easier to handle.

How does this work for wording?
```python
# Older Raspberry Pis should leave bus set at 0, while newer Pis should set
# bus equal to 1. The address should be changed to match the address of the
# sensor. (Common implementations are in README.md)
```