Add author information to posts in the RSS feed

feed.xml

@@ -19,6 +19,7 @@ layout: null
         <description>{{ post.excerpt }}</description>
         <pubDate>{{ post.date | date_to_rfc822 }}</pubDate>
         <link>{{ post.url | prepend: site.baseurl | prepend: site.url }}</link>
+        <author>{{ post.authors }}</author>
         <guid isPermaLink="true">{{ post.url | prepend: site.baseurl | prepend: site.url }}</guid>
         {% for tag in post.tags %}
           <category>{{ tag | xml_escape }}</category>