notes_for_developers.rst: relative import paths

This adds a mention of the manual replacement needed to adjust relative
paths due to a current protobuf limitation.
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passed cleanup-compose-test #672737237

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passed documentation #672737238

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passed e2e-tests #674595507

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passed type-check #672737234

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passed unit-tests #672737233

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failed e2e-tests #672737236

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