Add generic descriptions for templates

Fixes #8
diff --git a/templates/categories.html b/templates/categories.html
index e089140..7dfad91 100644
--- a/templates/categories.html
+++ b/templates/categories.html
@@ -1,6 +1,8 @@
 {% extends "base.html" %}
 {% block title %}Categories | {{ SITENAME }}{% endblock %}
 
+{% block description %}A listing of all categories on this site{% endblock %}
+
 {% block subheader %}
 {% include "modules/blogsubheader.html" %}
 {% endblock subheader %}