Extend repo to better track contributions and different versions

[cchwala]

• add CHANGELOG file
• add PR template with list of TODOs for each PR (also highlight the responsibility that comes with introducing breaking changes)
• figure out a way to better version the repo content (zenodo? or adding a version file directly to the repo?)
• extend prototype code that reads the ADOC files so that it takes versions into account

[cchwala]

Note that related (duplicated?) info is in #4