Change documentation to accommodate new SEMVER bump-keywords

[AndreasHassing]

With @faix's great work on the compliant pipelines, we'll need to change CONTRIBUTING.md to accommodate the new SEMVER bump-keywords which are:

• major version bump: +semver: breaking or +semver: major
• minor version bump: +semver: feature or +semver: minor
• patch version bump: +semver: fix or +semver: patch

For all three: the space in-between +semver: and the keyword is optional.

[AndreasHassing]

Need to find out if the +semver keyword needs to be in the PR title or if it's enough to have it in the commit text of the final PR merge commit.

[AndreasHassing]

Need to find out if the +semver keyword needs to be in the PR title or if it's enough to have it in the commit text of the final PR merge commit.

Do you know this, by chance, @faix?

[faix]

@AndreasHassing, in ADO I put it in the PR title. Since we usually squash, I think it’s better to put it in the PR title. Additionally it makes it easier when looking in the commit history to see when a breaking or minor change was made.