Describe the bug

While carrying out integration tests, I ran into a couple of very minor fixes that seem nonetheless important to correct.

• The contributors guidelines do not make explicit mention of venv creation. While anyone contributing is in theory aware of venv and its benefits, it's one of those things that have become so mechanical that one can easily forget. Best to add it.
• There is a typo in the logical_cnot example, probably left after some debugging. The num_workers is set to 20, whereas it probably should be set to cpu_count or not included at all being that it's an optional parameters that defaults to cpu_count

Steps to reproduce the behavior

Check the files linked above.

Expected behavior

Include venv creation in contributors guidelines.
Remove num_workers from logical_cnot example.

Python dependency versions

They're minor details. I will push a branch with fixes soon. I just didn't want to push a branch and then ask a merge without an issue, for clear historical documentation.