Jekyll post excerpt http://frontendcollisionblog.com/jekyll/snippet/2015/03/23/how-to-show-a-summary-of-your-post-with-jekyll.html

post excerpt.html

<!-- index.html -->
<p class="post-excerpt">

{% if post.content contains '<!--excerpt.start-->' and post.content contains '<!--excerpt.end-->' %}
	{{ ((post.content | split:'<!--excerpt.start-->' | last) | split: '<!--excerpt.end-->' | first) | strip_html | truncatewords: 20 }}
{% else %}
	{{ post.content | strip_html | truncatewords: 20 }}
{% endif %}

</p>

<!-- _posts/some-random-post.html -->
<p>
Here's all my content, and <!--excerpt.start-->here's where I want my summary to begin, and this is where I want it to end<!--excerpt.end-->.
</p>