Add info that python virtual environment package is reqd

[cfnz]

Had trouble installing Core and Ide, and it turns out that I did not have the python venv package installed.

This was not recommended anywhere. (Not sure if I have used the correct terminology, I am no Python expert.)

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1 out of 2 committers have signed the CLA.

:white_check_mark: cfnz
:x: ivankravets
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[ivankravets]

Thanks for the PR!