Put category description in meta description tag

app/controllers/list_controller.rb

@@ -74,6 +74,7 @@ class ListController < ApplicationController
       end
       guardian.ensure_can_see!(@category)
       list = query.list_category(@category)
+      @description = @category.description
     end
 
     list.more_topics_url = url_for(category_list_path(params[:category], page: next_page, format: "json"))

app/views/layouts/application.html.erb

@@ -4,7 +4,7 @@
     <meta charset="utf-8">
     <title><%= content_for?(:title) ? yield(:title) + ' - ' + SiteSetting.title : SiteSetting.title %></title>
     <meta content="width=device-width, initial-scale=1.0" name="viewport">
-    <meta content="" name="description">
+    <meta content="<%= @description %>" name="description">
     <meta content="" name="author">
 
     <%= canonical_link_tag %>