rtorrent: add note about clang failure

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

Formula/rtorrent.rb

@@ -10,6 +10,8 @@ class Rtorrent < Formula
   depends_on 'libtorrent'
   depends_on 'xmlrpc-c' => :optional
 
+  # Upstream says this is fixed in 0.9.x series, so check if this is
+  # still needed when the next stable rtorrent release is made.
   fails_with :clang do
     build 318
   end