Use proper release versions

We should version OAS properly, that is:

• Follow the semantic versioning guidelines
• Use versioned milestones instead of Feature-named ones (i.e. %"Documentation").
• Use signed git tags including the released version
• Do proper releases

I propose to use the %"Alpha-Test" as a first versioned release. But what version should we pick - 0.1.0-alpha1 ?

Release checklist:

• update “Known issues” section in README.md
• update/review documentation and make sure it matches the current state
• update CHANGELOG.md
• update the version number in the VERSION file
• create and push signed tag
• Make
• Announcements:
  • Blogpost
  • Infratest
  • … ?