git-flow: install completion into site-functions

`_git-flow` needs to be installed into `share/zsh/site-functions`
not `share/zsh/functions`.

Files in `share/zsh/functions` must not change unless zsh is
upgraded and zsh is restarted.  `site-functions` can change during a zsh
process's lifetime.

This will enable homebrew's zsh to automatically use the completion.
See issue Homebrew#11936 for details.

Closes Homebrew#11941.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Formula/git-flow.rb

@@ -22,7 +22,16 @@ def install
 
     GitFlowCompletion.new.brew do
       (prefix+'etc/bash_completion.d').install "git-flow-completion.bash"
-      (share+'zsh/functions').install "git-flow-completion.zsh"
+      (share+'zsh/site-functions').install "git-flow-completion.zsh"
     end
   end
+
+  def caveats; <<-EOS.undent
+    Bash completion has been installed to:
+      #{etc}/bash_completion.d
+
+    zsh completion has been installed to:
+      #{HOMEBREW_PREFIX}/share/zsh/site-functions
+    EOS
+  end
 end