Hints for rssguard in main

Warnings

• asv-tag-duplicated
  <code>com.github.rssguard.appdata.xml:31</code> - <em>content_rating</em><br/>This tag must only appear once in this context. Having multiple tags of this kind is not valid.
• translations-not-found
  Unable to add languages information, even though a <code>translation</code> tag was present in the MetaInfo file. Please check that its value is set correctly, and all locale files are placed in the right directories (e.g. <code>/usr/share/locale/*/LC_MESSAGES/</code> for Gettext .mo files).
• screenshot-download-error
  Error while downloading screenshot from 'https://github.com/martinrotter/rssguard/blob/master/resources/docs/images/rssguard-window.png?raw=true': URL was not found on the server.<br/>This might be a temporary server issue, or the screenshot is no longer available.