Systemd-Service: multiple processes parallel

[dzatoah]

I'd like to add documentation about this feature somewhere. What do you think is the best approach to document this @vfreex?

[vfreex]

Let's do it in README.md and -h option first.

[dzatoah]

So @vfreex, ready for review. :)

I didn't see the need to copy the same information also to the -h option…