Clarify usage instructions in PythonScripts/README.md

[moritz-gross]

No description provided.

[moritz-gross]

• pre release is skipped, so that part worked
• failing checks are unrelated to my changes I think:

error[E0599]: no method named join found for enum std::result::Result<T, E> in the current scope
--> src/bin/mathml2text.rs:89:84

• let's see if I can merge this once the checks pass.

[NSoiffer]

Someone else added a CLI interface and that was failing. I've fixed it up.

[moritz-gross]

ok, CI passes now.

I still can't approve the PR though. looks like CODEOWNERS (at least how I configured it here) is about who must review, not who can approve in the end. makes sense that those are two different things.

maybe still not bad to have, idk, but I think this is something you'd have to configure in the repo settings I guess.