Find a way to deal with old data models

[Jomula]

We should keep old data model schemas for being able to deal with data that has been contributed in an outdated model.
Possible ways to do that would be:

• Publish the Schemas as a release corresponding to their version number
• Establish an "archive" folder, where we can keep outdated schemas

Either way, we should maybe create a changelog for version changes.

[theodore-s-beers]

We can at least find the latest commit with a given version of the schema, and create a release from that.

Re: a changelog, do you prefer to have one document with a chronological list of versions and their changes? Or do you think it's ok to add some notes to each release on GitHub?