doctor: add instructions for remote migration.

Library/Homebrew/cmd/doctor.rb

@@ -777,6 +777,14 @@ def check_git_origin
         cd #{HOMEBREW_REPOSITORY}
         git remote add origin https://github.com/Homebrew/homebrew.git
       EOS
+    elsif origin =~ /mxcl\/homebrew(\.git)?$/ then <<-EOS.undent
+      Outdated git origin remote found.
+
+      The main Homebrew repository has migrated to:
+        https://github.com/Homebrew/homebrew.git
+
+      Run `brew update` (again) to migrate to the new main repository location.
+      EOS
     elsif origin !~ /Homebrew\/homebrew(\.git)?$/ then <<-EOS.undent
       Suspicious git origin remote found.