Add a README section about using Nix

[danieldk]

What does this PR do?

Describe how to get up-and-running with Nix.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

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