Gmail: update readme

[josephjclark]

@decarteret I've restructured and lightly redrafted the readme to make it a little clearer.

We use the word configuration in a very specific sense (as in, the configuration object on state), so I've edited carefully around the user of that word.

I've also tried to put broader introductory stuff up at the top of the readme to help orientation, with finer details further down.

Please merge this if you're happy!

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