Migration can not rolled back as sessions are now secure hashes. Than…

…ks to @n00dle.

Co-authored-by: n00dle <richardwu@gmail.com>

README.md

@@ -119,9 +119,13 @@ The following example Active Record Migration will work for the default setup of
 ```ruby
 # db/migrate/20210310083511_cve201925025_mitigation.rb
 class Cve201925025Mitigation < ActiveRecord::Migration[5.2]
-  def change
+  def up
     ActionDispatch::Session::ActiveRecordStore.session_class.find_each(&:secure!)
   end
+
+  def down
+    # no-op
+  end
 end
 ```