Added Beta Testing manual to Devdocs

What does this MR do?

This adds a (german) manual for beta testers to the dev docs. See Slack: https://yunity.slack.com/archives/C74JC2V26/p1581369354127500

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

Very

Links to related issues

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Follow instructions for setting up devdocs: https://devdocs.foodsharing.network/about-devdocs.html#setting-things-up
3. Go to http://localhost:4001/

Screenshots (if applicable)

Any relevant screenshots if this is a design / frontend change

Checklist

- [ ] added a test, or explain why one is not needed/possible...

- [ ] added an entry to CHANGELOG.md (description, merge request link, username(s))

• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• 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