update cookiecutter README.md instructions for code to work

[yannikbauer]

@klamann As we found out in the delivery.bootcamp 1:

1. In order to run 'pytest tests', the module code (src/) should be installed locally first, not later in the Notebooks section.
   If the package manager is pip, this can be done by running 'pip install -e .' (Needs to be adapted accordingly for poetry and conda).

2. I think another dependency that was not specified was jupyter lab.

3. Lastly, at least for PyCharm, the required Python interpreter setup should be mentioned. At least when using conda or pip, PyCharm does not know about the newly created virtual env (It might do when running poetry install but not tested).