Update docs to give the command to install test dependencies

[jaltekruse]

Summary

Small doc update, description pretty much covers it.

References

https://stackoverflow.com/questions/64685527/pip-install-with-all-extras

Reviewer guidance

Let me know if I missed something about how this should happen automatically?

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[rtibbles]

Looks right to me, thanks @jaltekruse!

[jaltekruse]

I was lazy and used the github UI to make this small edit. I won't do that in the future so I can run the lint check right away. The small second commit fixes the lint error.

[rtibbles]

Thanks @jaltekruse - I did a squash merge to collapse the commits in the history.