Add release notes to the snap and show them in Nextcloud via an app

[oparoz]

Users of the Box won't know what is happening to their system as they won't come to Github to look for a changelog they might not even understand. So, they might miss the introduction of new features.

We should create release notes, containing links to the wiki, when new features are introduced and these should be pushed via a release notes app.

Some documentation: https://github.com/nextcloud/notifications/blob/master/docs/notification-workflow.md

[kyrofa]

Yeah, great idea!

[github-actions[bot]]

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