Add Wiki for internal documentation

[phamkv]

It would be great to utilize the Wiki functionality in GitHub repo or do it elsewhere (look for alternatives). It can be informal (to store links and information related before they are forgotten) and can have a defined structure (a styleguide how wiki entries should look like).

[ChristianHuehn]

As discussed, we should do this only for concepts instead of code parts. We can use the project wiki for it. :) we could think of it as architecture documentation. We could use something like arc42 as format