I never knew about `update.rdf` and had to do some reading. How often is this used? Seems cool, but not really promoted.

What are the problems with having an updateURL and engine versions in the package.json? Isn't `update.rdf` returned by a server, so outside of the specific addon's working directory? Either way, the update.rdf is [not pretty](https://developer.mozilla.org/en-US/docs/Extension_Versioning,_Update_and_Compatibility#Update_RDF_Format) and something we should definitely migrate to a more modern format, like json.