Include a fix for pyenv with poetry shell in documentation

[eblume]

Please see #497, #571, #172, and possibly #198 for related issues.

In the case described by @itsthejoker here: #497 (comment)

a proposed solution is to have fish execute a script to fix $PATH after pyenv has initialized. I believe a better solution is to guard pyenv for $POETRY_ACTIVE. This PR includes documentation for this fix, as I suspect many people are hitting this roadblock.

I am not able to vouch that this fix works (or is needed) on any shells other than fish, but I would assume before merging you might want to include documentation for those shells.

Hopefully this is helpful!

[itsthejoker]

Oh. Thaaaat's a lot easier than my solution 😂

An important side note is that in Fish, this solution will only work if you're manually declaring the inits for pyenv inside your config instead of using the omf pyenv plugin (which helpfully mangles the path for us: https://github.com/oh-my-fish/plugin-pyenv/blob/master/init.fish). Might need an additional line in your update.

[eblume]

Please see #497 (comment) for a real fix.

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