Add description as comment on topic.

db/migrate/20170807082243_create_topics.rb

@@ -2,6 +2,7 @@ class CreateTopics < ActiveRecord::Migration
   def change
     create_table :topics do |t|
       t.string :title, null: false
+      t.text   :description_as_comment
       t.integer :author_id
       t.integer  "comments_count", default: 0
       t.references :community, index: true