Update README.md to put emphasis the possibility to supply arguments offered by client providers.

[TinQ0]

I was considering making a pr to share my addition to support the "hostedDomain" argument for Google OAuth when I finally read and understood the phrase stating "Each array entry is used as the configuration option to a new OAauth Provider Class instance so all options which are documented for the selected OAuth Provider class are available."

Stupid of me of course, but I think that expanding the example given does not unnecessarily complicate things, and that it may spare some future user of this plugin the time it took me to realise this.

Oh, and I standardised the capitalisation and comment spacing in that code block