Update contribution guidelines to non-zero major versions

[nipafx]

Closes #225.

Proposed commit message:

Update contribution guideline to non-zero major versions (#225 / #319)

The contribution guidelines mentioned that the major version "stays
at 0 for now". Change that to account for the looming 1.0 release,
after which new major version increases will be decided by the
maintainer.

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

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[Bukama]

LGTM 👍