Looks like it's only available as a full installer at the moment. It has appeared in Updates on my 13" rMBP running the 10.11.1 beta, but with a Download button (to download the full installer) instead of the usual Update."OS X v10.11 is already installed on this computer. Use the Updates page to install the 10.11 update or if you would like to download the full OS X installer click Continue."
But nothing shows up in software update![]()
Edit: Looks like the update version just hit.