Add an explicit CHANGELOG.md file

[netomi]

This PR adds an explicit CHANGELOG.md file with backfilled data from existing releases till v0.26.0.

The rationale behind such a file:

• have a central place where you indicate compatibility between different versions of released components
• clearly indicate the type of change (added, fixed, changed) to make it easier for operation people to review changes
• indicate what changes have been added to the main branch and are not released yet

This requires that PRs that get merged in also add a relevant entry in the changelog.