Calamari installation hints

[jbarth-ubhd]

Dear reader,
I would suggest adding the following installation hints:

virtualenv -p python3 calamari-0.3.5_venv
source calamari-0.3.5_venv/bin/activate
pip install calamari_ocr==v0.3.5
pip install tensorflow==v1.15.0   # v2.x does not work

[mikegerber]

This repository mainly serves as documentation of the provenance of the model published at https://qurator-data.de/calamari-models/, not as the definitive guide to training such a model.

I'd also argue that someone who would rather train a new model than just use the ready-made one, should be able to set-up a virtualenv and use pip install -r requirements.txt etc. So I'm not including any more installation hints to reduce maintenance work on the documentation, I hope you understand.

I did however update README.md to reflect this. 👍