jekyll setup

find related posts with category

{% for post in site.related_posts limit:5 %}
  {% assign match = false %}
  {% for category in post.categories %}
    {% if page.categories contains category %}
      {% assign match = true %}
    {% endif %}
  {% endfor %}
  {% if match %}
    <li><a href="{{ post.url }}">{{ post.title }}</a></li>
  {% endif %}
{% endfor %}

since site.related_posts limited by 10, using site.posts instead of site.related_posts as you wishes
@see: filter site related posts in jekyll

fency code block with raw tag

when running locally, if you included liquid template syntax like {% %} or {{ }}, just remember to embrace them with raw tag: {% raw %} ... {% endraw %}, or your code block would accidently broken.
@see: mojombo/jekyll#814

reference:

• GFM markdown help
• redcarpet2 help