Update the build instructions to handle updated dependency versions

[skypanther]

I had troubles building and discovered it was mostly a difference between dependency versions (Python version, Qt version). This PR updates the build instructions with details on what you need to tweak in order to get the app built.

Closes #225

[mkeeter]

Thanks for the PR! I'm confused as to whether editing CMakeLists.txt is actually necessary: when I build on my Mac, I see

-- Found Python: /usr/local/Frameworks/Python.framework/Versions/3.8/bin/python3.8 (found suitable version "3.8.5", minimum required is "3.3") found components: Interpreter Development Development.Module Development.Embed

implying that it's happy to select later Python versions if they exist.

What error messages were you seeing that led you to this solution?

[skypanther]

I get this:

(I have Qt 5.15.5 5.15.1, hence the updated cmake command below)

Console output removed because it works when you use the right version number. 🤦 I'll update the PR to remove instructions about editing the CMakeLists.txt file