[RFC] More instruction for testing metainfo.xml

[davidjharder]

In the case where packagers need to create or borrow metainfo.xml, and they want to validate it, we could provide better instructions.

Picking on getsolus/packages#1491 as an example:

I validate like so

appstream-util validate files/org.geonkick.geonkick.metainfo.xml
files/org.geonkick.geonkick.metainfo.xml: FAILED:
• tag-missing           : <content_rating> required [use https://odrs.gnome.org/oars]
• style-invalid         : <image> has vertical padding [https://geonkick.org/screenshot.png]
• style-invalid         : <image> has horizontal padding [https://geonkick.org/screenshot.png]
• tag-missing           : <release> required
• tag-missing           : <url> is not present
Validation of files failed

And validation has failed, but I think that check is too strict.

There is also:

appstreamcli validate files/org.geonkick.geonkick.metainfo.xml --pedantic
P: org.geonkick.geonkick:19: screenshot-no-caption
P: org.geonkick.geonkick:~: releases-info-missing
I: org.geonkick.geonkick:~: content-rating-missing

✔ Validation was successful: infos: 1, pedantic: 2

I think we can accept mateinfo.xml that passes the second check, correct?