Add some useful metadata

Any wording improvements are welcome :)

This passes the appstream validator now except for an info message

$ appstreamcli validate --explain org.postmarketos.Welcome.appdata.xml.in
I: org.postmarketos.Welcome.desktop:3: cid-contains-uppercase-letter org.postmarketos.Welcome.desktop
    The component ID should only contain lowercase letters.

Validation was successful: infos: 1