Add hint about address picker usage to zip code (PLZ) and city (Stadt/Gemeinde) fields

Closes #497 (closed)

What does this MR do?

Quick fix for #497 (closed) as discussed with @peter.toennies (https://yunity.slack.com/archives/C1T1ZM9A4/p1551008103213400)

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

95%

Links to related issues

https://gitlab.com/foodsharing-dev/devblog/issues/52, https://gitlab.com/foodsharing-dev/devblog/issues/57

Checklist

• added a test, or explain why one is not needed/possible...
• 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))