added a descriptive text to the photo uploader in settings.

What does this MR do?

New info box

How confident are you it won't break things if deployed?

Sure

Links to related issues

https://gitlab.com/foodsharing-dev/foodsharing-it-support/issues/123

Checklist

• added a test, or explain why one is not needed/possible... (just graphical)
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))