Added category layout

* see #11

Signed-off-by: Jim Martens <github@2martens.de>

_layouts/category.html

@@ -0,0 +1,15 @@
+---
+layout: default
+---
+
+{{ content }}
+<ul class="post-list list-unstyled">
+    {% for post in site.categories[page.category] %}
+    <li class="mb-4">
+        <span class="small text-muted">{{ post.date | date: "%b %-d, %Y" }}</span>
+        <p class="h4 post-title">
+            <a class="post-link" href="{{ post.url | absolute_url }}">{{ post.title }}</a>
+        </p>
+    </li>
+    {% endfor %}
+</ul>