Improve the installation instructions

[valeriupredoi]

Before you start, read CONTRIBUTING.md and the guide for diagnostic developers.

Please discuss your idea with the development team before getting started, to avoid disappointment later. The way to do this is to open a new issue on GitHub. If you are planning to modify an existing functionality, please discuss it with the original author(s) by tagging them in the issue.

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Tasks

• Create an issue to discuss what you are going to do, if you haven't done so already (and add the link at the bottom)
• Give this pull request a descriptive title that can be used as a one line summary in a changelog
• Circle/CI tests pass. Status can be seen below your pull request. If the tests are failing, click the link to find out why.
• Preferably Codacy code quality checks pass, however a few remaining hard to solve Codacy issues are still acceptable. Status can be seen below your pull request. If there is an error, click the link to find out why. If you suspect Codacy may be wrong, please ask by commenting.

If you need help with any of the tasks above, please do not hesitate to ask by commenting in the issue or pull request.

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Closes {Link to corresponding issue}

[bouweandela]

Looks good! Some minor comments about the last few lines.

[bouweandela]

Thanks for the updates! The explanation of pip vs setup.py is still not quite correct though (and maybe not needed either).

[bouweandela]

Thanks @valeriupredoi and @nielsdrost!