"API" documentation

[JBBianchi]

As we are thinking about letting people "implement" their own catalog, it might be a good thing to document how the catalog should work.

Few key points I have in mind right now:

• A function is accessed by "path", {function-name}/{version}/function.yaml
• Should the catalog provide an index ? This would allow the runtime to get the index and lookup the functions in there instead of "brute forcing" each function to lookup.
• Should we allow referenced resources (this is probably related to the specification ) ?
• Consider catalog API versioning (if in the future, we want to completely change the way the catalog is queried)