add post install message about semi-backwards incompatibility

Author: oreoshake

secure_headers.gemspec

@@ -19,4 +19,5 @@ Gem::Specification.new do |gem|
   gem.test_files    = gem.files.grep(%r{^(test|spec|features)/})
   gem.require_paths = ["lib"]
   gem.add_development_dependency "rake"
+  gem.post_install_message = "Warning: lambda config values will be broken until you add |controller|. e.g. :enforce => lambda { |controller| some_expression }"
 end