Mention that debugging can be slow.

https://github.com/rails/rails/pull/3180\#issuecomment-14705821 has
mention of rendering going from 1-2ms to 4 seconds with this on,
which seems reasonable: debugging is slow.

railties/lib/rails/generators/rails/app/templates/config/environments/development.rb.tt

@@ -26,6 +26,8 @@
 
   <%- unless options.skip_sprockets? -%>
   # Debug mode disables concatenation and preprocessing of assets.
+  # This option may cause significant delays in view rendering with a large
+  # number of complex assets.
   config.assets.debug = true
   <%- end -%>
 end