[Toaster] Adding Toaster to the quick start guide

[Barros Pena, Belen]

Call me crazy, but I think inviting people to set up Toaster once they
complete the Yocto Project set up in the Quick Start guide seems a good
idea to me. 

I guess it could fit between the "Yocto Project release" and the "Building
images" sections. Would you be terribly opposed to this? Or does this
sound like something we could do?

Thanks!

Belén