Osxfuse: Unsigned Kexts Caveat

Related to [this issue](https://github.com/Homebrew/homebrew/issues/31164). Adds a warning in the “Caveats” that once Yosemite is released it is likely you’ll need to disable the kext check mechanism for OSXFuse to successfully run.

I have an open bug/feature report/request with Apple upstream to ask whether they'll include a way to enable or disable individual kexts rather than either allowing them all to run or none, but I don't expect major changes at this point.
I'm open to the idea of ditching OSXFuse support from source for 10.10. We should probably try and remove some of the Doctor warnings about it installing into the Homebrew prefix with the binary though, if that's the route we take.

Yeah, I'm not fond at all of the new Apple rule of "Run all unsigned kexts or none at all". It's frustrating. I've upstreamed that issue but I don't expect to see it changed for 10.10 at this point.