add text about refactoring to devdocs

Closes None

What does this MR do?

Adds a text about refactoring to the devdocs

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

It is just a devdocs article, please look over if it is correct :)

Links to related issues

Was discussed during https://yunity.slack.com/archives/CD7HJ1HHC/p1587823492001900 https://codi.kanthaus.online/2020-04-25_code_refactoring?both#

How to test

See https://devdocs.foodsharing.network/about-devdocs.html#setting-things-up

Screenshots (if applicable)

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))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings