converting data to a numpy array

The docs would be *much* more useful to me if this introductory sample
```
with audioread.audio_open(filename) as f:
print(f.channels, f.samplerate, f.duration)
for buf in f:
do_something(buf)
```
included some notion of how to convert `buf` to a numpy array. If you have a minute. (and, if that's easy to do)

Locate the introductory audioread sample containing `audio_open` and the `for buf in f` loop, then read the surrounding documentation to understand the buffer format. Document how a user can convert `buf` to a NumPy array, and confirm that the sample clearly explains the resulting data and remains consistent with the existing API.

Written by the indexing model from the issue text.