Describe about RUBY_CODESIGN

	With RUBY_CODESIGN people can avoid pushing many OK button of firewall
	dialogs on macOS through test-all.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57884 b2dd03c8-39d4-4d8f-98ff-823fe69b080e