Define and document a learning-path for contributors to the PLC part

[zantiu]

The learning path should get new contributors up and running as quickly as possible.
It should also document common issues.

Siemens already has a recommended learning path, so suggest to refer to that as a first step:
https://github.com/simatic-ax/learning-path

I just tried to run through it but got stuck at HW configuration download step, which is not so well documented.
I opened an issue here:
simatic-ax/learning-path#11