usage.md: warning on --caskroom

Closes Homebrew#10009.

USAGE.md

@@ -180,7 +180,8 @@ $ brew cask install caskroom/fonts/font-symbola
 You can also modify the default installation locations used when issuing `brew cask install`:
 
 * `--caskroom=/my/path` determines where the actual applications will be located.
-Default is `/opt/homebrew-cask/Caskroom`
+Should be handled with care — setting it outside `/opt` or your home directory might mess up your system.
+Default is `/opt/homebrew-cask/Caskroom`.
 * `--appdir=/my/path` changes the path where the symlinks to the applications (above)
 will be generated. This is commonly used to create the links in the _root_ Applications directory
 instead of the _home_ Applications directory by specifying `--appdir=/Applications`. Default is `~/Applications`.