Update database.md

What does this MR do?

Note how and where to create sql scripts

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

useful for newbies

Links to related issues

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))