Versioning and changelog

Why

• currently, the version number is not being incremented. This makes it hard to consume the library.
• currently, there is also no changelog, making it risky to update
• the master release NuGet package (without the commit id) is overwritten every time the pipeline runs on the master branch

Backwards-compatible changes

• a CHANGELOG.md is introduced
• the MR template is changed to distinguish between compatible/incompatible changes

Incompatible changes / side effects

• future MRs should update the CHANGELOG.md
• the .gitlab-ci.yml is changed such that the release-nuget-master job is conditioned on version-tagged commits on the master branch. This way it will only run when a Tag (named like v1.2.3) is created under Repository/Tags.
• Maintainers will be in charge of incrementing the version number & making releases:

1. increment the version number of the library (remain with 0.x.x until official release)
2. update the CHANGELOG.md with the current date (and creating a new [vNext] section)
3. commit & push to master
4. create a Tag under Repository/Tags, using the respective section from the CHANGELOG.md for the release notes

Checklist

• added relevant changes to the CHANGELOG.md in the [vNext] section
• closes #32 (closed)