Add meta tags for a better SEO
Fix missed new line tags
Add the Comment model to the back-end section

Since there is no way to delete comments directly from the front-end
a single resource registering with ActiveAdmin is enough. By the way
ActiveAdmin defines a Comment model so we just use a little trick with
the :as option to prevent an error to be triggered
Add a little hint for comments form

Add a little hint with a link to the Markdown reference to tell users
that comments a parsed through Markdown