Added call for contribution (ger) to devdocs

Almost finished there - for my part. Let's see how it looks.
Please don't delete this branch afterwards.
You can still change the text or typos via the pad.systemli.org link I posted everywhere.

Please hit that merge button :)

Edit: Reasons for me to add it to devDOCS instead of devBLOG is cause it's a document we should continuously improve and update. It's very similar to the tech documentation on devDOCS: Everyone can add and change details.
A BLOG is something bound to a date which is even in the link. This would feel outdated even if we keep the text there up to date