build: add citation needed

for the bit

build.pl

@@ -74,6 +74,10 @@ sub post_to_meta {
             "<a class=\"fn\" onclick=\"fnClick(this)\" name=\"fn-$1\" href=\"#fn-$1-text\">[" . $ifn . "]</a>"
         }egm;
 
+        $content{"body"} =~ s{\$cn\$}{
+            "<a class=\"fn\">[citation needed]</a>"
+        }egm;
+
         $metadata{content} = $parser->markdown_to_html(
             dialects => 'GitHub',
             markdown => $content{"body"}