Write a how-to on how to use the asSystemProxy option

[waldekmastykarz]

Ah thank you, we tried that and that did the trick :)
But unfortunately this option is missing in the schema and therefore not being suggested with auto-completion. I guess we overread the property in devproxy -h, but at least it is listed there.
Suggestion to maybe add it to the Getting Started section, since otherwise it is also pretty hidden in the Documentation :)

Originally posted by @coinzz in #917