Included venv instructions and added venv/ to .gitignore

[shadow00]

By default, running pip3 install -r requirements.txt outside of a virtual environment will install packages for the entire system. This is bad, since different projects may require conflicting versions of the same packages. The best practice is to use some kind of virtual environment (this feature typically comes preinstalled with most distributions, so all you really need to do is run a simple command to set it up) and install the packages inside that venv.

This also makes it much easier to contribute for anyone who's already familiar with using a venv.

Note that this PR will create a merge conflict with #125 because they both change the contents of the README.md file. I suggest merging this PR first, and then I'll rebase and force-push my config branch for the other PR - that way all you'll need to do is just merge them both.

[Datalux]

@shadow00 thank you very much for your work!
I had planned to add a virtual environment for this project but you preceded me by implementing all things.
I'm very happy!