node: Add comment explaining why --without-npm

Users looking at the formula don't know why we are not installing npm.
Add a comment pointing to the original discussion.

Closes Homebrew/homebrew#8989.

Signed-off-by: Charlie Sharpsteen <source@sharpsteen.net>

Formula/node.rb

@@ -24,6 +24,7 @@ class Node < Formula
       s.gsub! '/opt/local/lib', '/usr/lib'
     end
 
+    # Why skip npm install? Read https://github.com/mxcl/homebrew/pull/8784.
     args = ["--prefix=#{prefix}", "--without-npm"]
     args << "--debug" if ARGV.include? '--debug'