FEATURE: Add CommentMigration for db column comments

For documenting the database (will be used in discourse-data-explorer)

db/migrate/20150727230537_add_example_column_comments.rb

@@ -0,0 +1,26 @@
+require 'comment_migration'
+
+class AddExampleColumnComments < CommentMigration
+
+  def comments_up
+    {
+      posts: {
+        _table: 'If you want to query public posts only, use the badge_posts view.',
+        post_number: 'The position of this post in the topic. The pair (topic_id, post_number) forms a natural key on the posts table.',
+        raw: 'The raw Markdown that the user entered into the composer.',
+        cooked: 'The processed HTML that is presented in a topic.',
+        reply_to_post_number: "If this post is a reply to another, this column is the post_number of the post it's replying to. [FKEY posts.topic_id, posts.post_number]",
+        reply_quoted: 'This column is true if the post contains a quote-reply, which causes the in-reply-to indicator to be absent.',
+      },
+      topics: {
+        _table: "To query public topics only: SELECT ... FROM topics t LEFT INNER JOIN categories c ON (t.category_id = c.id AND c.read_restricted = false)"
+      },
+    }
+  end
+
+  def comments_down
+    {
+    }
+  end
+
+end