I think there are two different types of style guides being discussed here, and I usually cover both types in my own internal corporate style guides. One is a general writing guide (such as that which you’d find in the Chicago guide, or even Strunk & White). The other is a technology style guide that we use to help us when we document software or hardware.
That said, I also use Apple’s style guide (if you are writing for Mac or iOS products, this guide is invaluable.
And, as I commented in the thread from which this one was branched, the Sun style guide is no longer in existence, as Sun was bought out by Oracle and I don’t believe Oracle updates that doc any longer. In technology years, the six years since “Read Me First!” was last updated is a lifetime ago. Particularly if you stop to think about documenting new OSes such as Windows 8.1 and tablets, with swipes and tiles (and no real Start menus!).
P.S. Just looked, and while open source, the GNOME guide might be even older that “Read Me First!”