Add comments controller and routes

app/controllers/comments_controller.rb

@@ -0,0 +1,11 @@
+class CommentsController < ApplicationController
+  def index
+    comments = Comment
+      .where(post_id: params[:post_id])
+      .left_outer_joins(:user)
+      .select('comments.body, comments.updated_at, users.full_name')
+      .order(updated_at: :desc)
+
+    render json: comments
+  end
+end