_includes/related.html
<div class="related">
{% assign hasSimilar = '' %}
{% for post in site.posts %}
{% assign postHasSimilar = false %}
{% for tag in post.tags %}
{% for thisTag in page.tags %}
{% if postHasSimilar == false and hasSimilar.size < 5 and post != page and tag == thisTag %}
{% if hasSimilar.size == 0 %}
<h4>Related Posts</h4>
<ul>
{% endif %}
<li class="relatedPost">
<a href="{{ site.url }}{{ post.url }}">{{ post.title }}
{% if post.series %}
(Series: {{ post.series }})
{% endif %}
</a>
</li>
{% capture hasSimilar %}{{ hasSimilar }}*{% endcapture %}
{% assign postHasSimilar = true %}
{% endif %}
{% endfor %}
{% endfor %}
{% endfor %}
{% if hasSimilar.size > 0 %}
</ul>
{% endif %}
</div>