[FIX] README: Add Script Mode configuration to Quickstart Guide

[sbutzek]

I had some trouble the other day setting up karma-ui5 in script mode.
I was missing a minimum running example for script mode.
After half a day try-and-error, I found one.
Maybe you also find this helpful for the readme.

I also moved the "mode" section out of "Options" because I think is is a crucial option that stands above all the other options.

[CLAassistant]

All committers have signed the CLA.

[matz3]

Thanks @sbutzek 👍🏻
I agree that an example for the script mode was missing.
Instead of documenting the example below the script mode option I've added it to the "Quickstart" guide.
Any feedback welcome!

[sbutzek]

Fine for me, thank you @matz3 !