Add QUERY example to introduction

[mnot]

Fixes #2171

[reschke]

Yes, but.

If we do this, we'll also need to define the QUERY semantics for this content type, no?

Should I give that a try?

[mnot]

That'd be good -- I'm curious to see how that works :)

[sandstrom]

Out of curiosity, what's the status for this (not this issue, but QUERY in general)?

I think QUERY is a great addition to the HTTP spec, and would love to start using it.