Adapt the README for PyPI

[nolar]

Issue : #12

PyPI now renders the README (long_description) improperly. It needs some hinting on the content type.

The links must be absolute, as they are shown not only in GitHub anymore.

The "Documentation" link on PyPI was pointing to README, but we now have the full docs.

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[zincr]

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❌ Approvals
✅ Large Commits
✅ Specification
✅ Dependency Licensing

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• ✅ Approved by PR author @nolar
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