FAQ: Add cached downloads directory for Linuxbrew

Author: sjackman

share/doc/homebrew/FAQ.md

@@ -68,7 +68,7 @@ Be careful as this is a destructive operation.
 
     brew --cache
 
-Which is usually: `/Library/Caches/Homebrew`
+Which is usually: `/Library/Caches/Homebrew` on Mac and `~/.cache/Homebrew` on Linux
 
 ### My Mac `.app`s don’t find `/usr/local/bin` utilities!
 GUI apps on OS X don’t have `/usr/local/bin` in their `PATH` by default.