[#221] Explain how to write Show instances

[gromakovsky]

Description

Problem: if you manually define Show instance and switch to Universum, they will fail to compile. It's a gotcha we don't mention.

Solution: mention it in gotchas and the migration guide.

Related issues(s)

Resolves #221

✓ Checklist for your Pull Request

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Related changes (conditional)

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[neongreen]

what a holy day