Add atom link

templates/posts.html

@@ -3,6 +3,8 @@
 {% block main_content %}
 {{ post_macros::page_header(title="blog posts") }}
 
+<p>if you want to subscribe to updates, <a href="/atom.xml">an atom feed</a> is available.</p>
+
 <div class="list">
   {%- if paginator %}
   {%- set show_pages = paginator.pages -%}