Document changes of the `format-version` in a central place

We currently have:

• When a new feature is added, a new string ``The configuration is available since format-version X```
• Buildstream projects need to specify the format-version in their projects

Still it would be good to have a central place to document what new features are being added in every bump, maybe a good location would be http://buildstream.gitlab.io/buildstream/projectconf.html#format-version ?