Doc for data read

[ChristophSchranz]

First of all: Thank you very much for this great sensor standard library. I think this one is perfect for multi-sensor measurements.

One point that was not straight head for me (as I used not the best IDE) was to read the data.
Maybe you have some time to update the README with an example of how to read some measurement from e.g. a Movisens device.

u.entries
u.ecg_bin.attrib
u.ecg_bin.get_data()