Clearify new store business chain description

This MR closes nothing

What does this MR do?

Clearify the description, to remember the store-chain-rules if you want to create a new store. The actual text could easily be missunderstood.

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

shure

Checklist

• added a test, or explain why one is not needed/possible... Its only text
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• no need to add an entry to CHANGELOG.md, i think.

6 from 10 voted for the changing. (Screenshot from the workinggroup of the business-chain admins)