Are Features part of the Archimate exchange file format standard? I'm assuming yes since XML is mentioned.
I also see Features on the coArchi roadmap. This makes me wonder if this could be used in substitution of the ReadMe.md that is generally used with Git.
No. A "feature" is simply an internal dictionary entry (key/value pair) like a Property.
Before now, every time we added an internal attribute to a model element (for example, colour, opacity, size, whatever) we would have to add a new attribute to the Eclipse Modelling Framework (EMF) eCore Model and regenerate the Java code. This also meant that a newer model could not be opened in an older version of Archi.
A "feature" stores these internal attributes in a generalised way. They should only be exposed in the UI at the appropriate place, and not like a Properties table. For example the outline opacity of a diagram element is stored in a feature like this:
<feature name="lineAlpha" value="23"/>
and appears in the UI as a number widget (0-255).
As I said, these "features" are for internal use and should be exposed somehow to the user through the UI. Exposing it in the UI means that the user can remove it (by setting it to its default value) otherwise it will stay there forever.
A "feature" should not be used for storing arbitrary data unless the user or the plug-in can remove it.
This has nothing to do with the ArchiMate exchange file format. Take the example above. Internally we store outline opacity in a feature, but when we export to the exchange format it is stored in a dedicated part of the XML format (I think it is "alpha"). So, totally different things.
Phil