Update `CONTRIBUTING.md`

[sebastian-peter]

Similar to ie3-institute/simona#201

[pierrepetersmeier]

@sebastian-peter

The JavaDoc (line 19) link is no longer working. Does this refer to a link to a Java Documentation or the JavaDoc software itself?
And Are they still involved in the project (@sensarmad,@ckittl)?

[sebastian-peter]

I'd consider taking care of JavaDoc as part of this, so maybe just remove the mentioning here.

As to the mentioning of authors, I'd remove the list in CONTRIBUTING.md and link to an (updated) AUTHORS file. There, please also add @danielfeismann, @SimonHuette, @jo-bao, @julianhohmann and yourself to the list of main contributors and Thomas, Daniel, Johannes Bao and me to the list of lead devs. Hope that I have not forgotten anyone.