add QML grammar to documentation

We discussed this on the qt-qml mailing list some time ago and I've now found the time to clean it up: a nice, human-readable and hopefully correct QML grammar is attached. Ideally the productions would have explanations and links to other parts of the documentation, but I think it's already helpful in the current form and should be included in the documentation.

Aaron, I'm assigning this to you for now since you showed interest on the mailing list. It should probably go to someone responsible for the QML documentation though.