docs: add parameters from sample screenshot

[agorgl]

Currently the main screenshot in the README does not match the default configuration of tuigreet.
The screenshot seems a bit more beautiful than the defaults, so it would be nice if we specified the arguments that reproduce it.

I suppose it is something along the lines of:

tuigreet --time --asterisks --prompt-padding 0 --window-padding 1

along with some --theme ??? parameter that makes the background bluish instead of black.