Add categories to config.yml for rss

Add categories so that (hopefully) the
rss feed can be categoriezed readers will
be able to select the type of post (new, blogs, talks, etc)
that they'd like to see on their feed.  This
will hopefully help with containers#213

Signed-off-by: TomSweeneyRedHat <tsweeney@redhat.com>

_config.yml

@@ -18,6 +18,13 @@ readme_index:
 
 excerpt_separator: "<!--readmore-->"
 
+feed:
+  categories:
+    - blogs
+    - new
+    - releases
+    - talks
+
 authors:
   adrianr:
     name: Adrian Reber