[vbox-dev] I think we should update User's Guide about Mac OS X

[Alexey Eromenko]

Installing VirtualBox on Mac OS 10.13 host ("High Sierra") requires
new steps, which is not mentioned in our user's guide.

I got a new Macbook 12" to play with and was unable to install latest
VBox 5.2.2, until I found this:
https://apple.stackexchange.com/questions/300510/virtualbox-5-1-8-installation-didnt-install-kernel-extensions-how-do-i-fix-thi

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-Alexey Eromenko "Technologov"