Mention OS X browser in FAQ

Closes #255.

README.markdown

@@ -86,12 +86,13 @@ directory.  Edit a file from the repository.
 Fugitive cares about the current file, not the current working
 directory.  Edit a file from the repository.
 
-> Why is `:Gbrowse` not using my system default browser?
+> Why is `:Gbrowse` not using the right browser?
 
 `:Gbrowse` delegates to `git web--browse`, which is less than perfect
-when it comes to finding the default browser on Linux.  You can tell it
-the correct browser to use with `git config --global web.browser ...`.
-See `git web--browse --help` for details.
+when it comes to finding the right browser.  You can tell it the correct
+browser to use with `git config --global web.browser ...`.  On OS X, for
+example, you might want to set this to `open`.  See `git web--browse --help`
+for details.
 
 > Here's a patch that automatically opens the quickfix window after
 > `:Ggrep`.