Preferences UI is undocumented

I just switched to installing Guake via pip instead of via my system package manager (since the latter is still on 0.8.4). Unfortunately, now it's not clear how to access the preferences UI; when Guake was installed via apt it created a desktop entry for this, but the install via pip doesn't seem to do anything of the sort.

I eventually discovered ([here](https://github.com/Guake/guake/issues/1117), via searching issues) that it's possible to run `guake -p` to access the preferences, but it would be really nice if that was explained somewhere more obvious in the docs. (I see now that it's listed under `Command Line Arguments`, but I only looked there _after_ learning that the way to access the prefs UI was via a CLI switch -- I would never have thought to look there first!)