Add example layout files

[ronnyhartenstein]

The plugin uses category_list.html, category_index.html and category_feed.xml.
It would be really great for onboarding to have a template for that.

[alain-andre]

One should basically be

<!-- _layouts/category_index.html -->
<ul>
{% for post in site.categories[page.category] %}
  <li><a href="{{ root_url }}{{ post.url }}">{{ post.title }}</a></li>
{% endfor %}
</ul>