Improve documentation: Datalogger only

[denniswo]

If my understanding is correct, I believe the following changes would improve the installation instructions for the datalogger only combined with a fully installed instance of DSMR-reader.

1. On the datalogger only installation:
   The installation instruction:
   sudo sh -c 'echo "cd ~/dsmr-reader" >> /home/dsmr/.bashrc'
   appears unnecessary, because this directory is never created. It results is a (harmless) error.

2. On the fully installed installation:
   I believe the installation instruction:
   sudo cp /home/dsmr/dsmr-reader/dsmrreader/provisioning/supervisor/dsmr_datalogger.conf /etc/supervisor/conf.d/
   should be omitted, because this instance will not do any data logging.
   (In my case the VM I installed it in didn´t even have a USB port, so supervisor kept trying to start this task in vain.)

Background:
I was already forwarding my telegrams from a RPi3 installation to a home server, but the RPi3 installation was a fully installed instance.
As part of my recent upgrade from DSMR-reader v2 to v3, I made the RPi installation a datalogger only instance. I even downgraded the RPi3 to a RPi1 and this is indeed enough to keep up with the data stream from a DSMR 5.0.
This is working very well. Thanks. Big fan.

[dennissiemensma]

Thanks, I've clarified the docs in the next release.