Comment # 2 on bug 1013292 from Atri Bhattacharya

Apps that do not install a correct appdata into the proper directory
(/usr/share/appdata/) do not show up in gnome-software (some common issues with
appdata files can be seen at [1]). This is by design. If you want any app to
show up in the software app, please file bugs against specific apps (e.g.
Chromium), preferably upstream.

[1] http://gs-stats.leuenberger.net/appdata-failed.html