vcs-browser URL type in metadata.xml

freedesktop.org specifies the content of metadata.xml in [this document](https://freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-url). In the url/ part, the specification lists a URL type 'vcs-browser':

`vcs-browser

URLs of this type should point to a webpage on which the user can browse the sourcecode. `

I tried to include this file in my metadata.xml, but it failed the validation. Maybe it's a new addition to the list of URL types, and the validation code is not yet ready to accept it? Including it actually fails the validation, so maybe code is needed to ignore it, if it's unwanted.