Disqus comment form can be embedded with following steps.

  1. Get “Universal Code” from Disqus admin page.

  2. Put the html code into _includes/disqus.html file.

  3. Add {% include disqus.html %} at bottom of _layouts/post.html.

_includes/disqus.html

<div id="disqus_thread"></div>
<script type="text/javascript">
/* * * CONFIGURATION VARIABLES: EDIT BEFORE PASTING INTO YOUR WEBPAGE * * */
var disqus_shortname = 'y15e'; // required: replace example with your forum shortname

/* * * DON'T EDIT BELOW THIS LINE * * */
(function() {
  var dsq = document.createElement('script'); dsq.type = 'text/javascript'; dsq.async = true;
  dsq.src = '//' + disqus_shortname + '.disqus.com/embed.js';
  (document.getElementsByTagName('head')[0] || document.getElementsByTagName('body')[0]).appendChild(dsq);
})();
</script>
<noscript>Please enable JavaScript to view the <a href="http://disqus.com/?ref_noscript">comments powered by Disqus.</a></noscript>