Update CONTRIBUTING.md

[nohli]

Description

Update the changelog to make it more clear that PRs should update the version number.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[vbuberen]

Maybe we could also mention how we approach versioning?
This link should work, I believe: https://semver.org/

[alexbatalov]

@nohli Please also update PR template, as now it is clearly states :

Updating the pubspec.yaml and changelogs is not required.

[nohli]

@alexbatalov good finding, done

[mhadaily]

LGTM! thanks