[JOSS REVIEW] Add better instructions on how to run your tests

[finsberg]

You write in you README file that to run the tests you only need to install pytest, but it will not work without installed additional dependencies. This is in part addressed in #234, but the instructions in the README should be updated.

This is related to the ongoing review at openjournals/joss-reviews#8767

[adperezm]

Thank you for the contribution @finsberg. It is appreciated.

We have now modified the instructions to allow users quickly install everything that is required on a fresh environment.