add info about twitter cards..

BLOG.md

@@ -29,3 +29,12 @@ parent level via:
   some lines
   ````
 ```
+
+If you want to check twitter cards, use the url: https://cards-dev.twitter.com/validator
+
+The following additional meta data maybe added:
+twittertitle: Use this instead of title
+image: URL of an image to show along with the post
+imagealt: Alt text for the image
+
+[twitter card reference](https://developer.twitter.com/en/docs/twitter-for-websites/cards/overview/markup)

layout/base.j2

@@ -33,6 +33,7 @@
 {% endif %}
 <meta property="og:description" content="{{ resource.meta.description }}" />
 {% if resource.meta.image %} <meta property="og:image" content="{{ resource.meta.image }}" />{% endif %}
+{% if site.context.tweet_via %} <meta property="twitter:site:id" content="{{ site.context.tweet_via }}" /> {% endif %}
 {% if resource.meta.imagealt %} <meta name="twitter:image:alt" content="{{ resource.meta.imagealt }}" />{% endif %}
 {% if resource.meta.twitterlabel1 %} <meta name="twitter:label1" content="{{ resource.meta.twitterlabel1 }}" />{% endif %}
 {% if resource.meta.twitterlabel2 %} <meta name="twitter:label2" content="{{ resource.meta.twitterlabel2 }}" />{% endif %}