Update seo.html for meta description (mmistakes#558)

Adding standard meta description immediately after seo_description has been defined

_includes/seo.html

@@ -25,6 +25,8 @@
   {% assign seo_description = seo_description | markdownify | strip_html | strip_newlines | escape_once %}
 {% endif %}
 
+<meta name="description" content="{{ seo_description }}">
+
 {% assign seo_author = page.author | default: page.author[0] | default: site.author[0] %}
 {% if seo_author %}
   {% if seo_author.twitter %}