Help messages for new contributors

[raphodn]

Story

Display short helpful messages in known areas where they might struggle

• searching for a location
  • [x] help messages when 0 results are returned
• uploading a proof
• contribute / experiments / action cards
  • more context

[Showmaster3000]

Another point is the proof. I just started using open prices some weeks ago and later found a documentation somewhere in git or so how the creators want the proofs - for example: price tag plus product. This is not mentioned anywhere. Or just not important anymore...