#542 Add first draft for chronos-multithreading docs

[PhilippMDoerner]

First draft for what I learned with chronos in multithreading so far and how that could be turned into docs as discussed in #542 .

If you had a different vision @arnetheduck let me know, I'll see what I can do.

Note that I think a rather obvious question that I do not yet know the answer for is:
"What do I do when I want the thread to end, but the Dispatcher still has work on it? How do I tell it to process the remaining work?"

If I had an answer to that I'd gladly add an example to the docs.