Add Short Summary to Blog Posts

add a quick one or two line summary of the post for aggregation.  This should be in the following tags

``` html
<details>
 <summary>Summary of the article</summary>
</details
```

This is described here: https://developers.facebook.com/docs/instant-articles/guides/articlecreate

This should also be marked up with the **headline** tag from schema.org (see https://schema.org/BlogPosting)
