rss: add an updated date

- Adds an uptdated date to the item feed.

Co-Authored-by: iGor milhit <igor@milhit.ch>

layouts/_default/rss.xml

@@ -30,6 +30,9 @@
       <title>{{ .Title }}</title>
       <link>{{ .Permalink }}</link>
       <pubDate>{{ .PublishDate.Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</pubDate>
+      {{ if (and (isset .Params "lastmod") (gt (dateFormat "2006-01-02" (.Lastmod)) (dateFormat "2006-01-02" (.PublishDate)))) }}
+      <updated>{{ .Lastmod.Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</updated>
+      {{ end }}
       {{ with .Site.Params.author.email }}<author>{{.}}{{ with $.Site.Params.author.name }} ({{.}}){{end}}</author>{{end}}
       <guid>{{ .Permalink }}</guid>
       <description>{{ .Content | html }}</description>