Add Docs about how to build python libraries

[derDieDasJojo]

Problem
If I try running python3 ./application/romi-python/main.py as described in: https://docs.romi-project.eu/Rover/software/ I run into the following error message

python3 main.py 
Traceback (most recent call last):
  File "~/romi/applications/romi-python/main.py", line 4, in <module>
    from romi.rpc import Server
ModuleNotFoundError: No module named 'romi'

so I first of all have to build and install the python library:

cd libromi/python
python3 -m pip install -e .

I think this step should be added to https://docs.romi-project.eu/Rover/software/#starting-up-the-software-manually