Add hreflang metadata to page header (#374)

layouts/partials/header.html

@@ -9,6 +9,11 @@
   {{ .Hugo.Generator }}
   {{ with .Site.Params.name }}<meta name="author" content="{{ . }}">{{ end }}
   {{ with .Site.Params.role }}<meta name="description" content="{{ . }}">{{ end }}
+  
+  {{ range .Translations }}
+  <link rel="alternate" hreflang="{{ .Lang }}" href="{{ .Permalink }}">
+  {{ end }}
+  <link rel="alternate" hreflang="{{ .Site.LanguageCode | default "en-us" }}" href="{{ .Permalink }}">
 
   {{ $sri := .Site.Data.sri }}
   {{/* Default to enabling highlighting, but allow the user to override it in .Params or .Site.Params.