Thanks for adding the bundle tag and for adding AppImage support.

Please document the best practices for how the bundle tag should be used. I try to summarize what I understood from our earlier conversation, feel free to correct me:

• The bundle tag is not supposed to be added to usr/share/metainfo/$ID.appdata.xml files but is added to collection data (a.k.a. distro metadata) by a compose server (e.g., operated by a distribution or app store). If upstream adds the bundle tag to usr/share/metainfo/$ID.appdata.xml anyway it is likely to be removed by the compose server
• The compose server has to get the information from upstream. Upstream can submit it to the compose server by either placing a file in a special format (still to be defined) into a upstream-provided directory; or by using an upload tool (still to be written) that submits the data to the compose server; or by manually enter the information into a web interface of the compose server (to be written)
• It is not allowed to have sub-tags inside the <bundle> tag. This is to allow for easier verification and deserialization. But it is possible to have more than one <bundle> tag, even for the same bundling format (this is useful when an upstream wants to provide different channels like stable, weekly, daily, continuous, canary, aurora, etc.)