add lastmod and images field to metadata and seo

data/blog/introducing-tailwind-nextjs-starter-blog.mdx

@@ -1,9 +1,11 @@
 ---
 title: 'Introducing Tailwind Nexjs Starter Blog'
 date: '2021-01-12'
+lastmod: '2021-01-18'
 tags: ['next-js', 'tailwind', 'guide']
 draft: false
 summary: 'Looking for a performant, out of the box template, with all the best in web technology to support your blogging needs? Checkout the Tailwind Nextjs Starter Blog template.'
+images: ['/static/images/canada/mountains.jpg', '/static/images/canada/toronto.jpg']
 ---
 
 ![tailwind-nextjs-banner](/static/images/twitter-card.png)
@@ -88,7 +90,39 @@ You can start editing the page by modifying `pages/index.js`. The page auto-upda
 
 `pages` - pages to route to. Read the [Next.js documentation](https://nextjs.org/docs) for more information
 
-## Compose
+## Post
+
+### Frontmatter
+
+Frontmatter follows [Hugo's standards](https://gohugo.io/content-management/front-matter/).
+
+Currently 7 fields are supported.
+
+```
+title (required)
+date (required)
+tags (required, can be empty array)
+lastmod (optional)
+draft (optional)
+summary (optional)
+images (optional, adds to socialBanner in siteMetadata config)
+```
+
+Here's an example of a post's frontmatter:
+
+```
+---
+title: 'Introducing Tailwind Nexjs Starter Blog'
+date: '2021-01-12'
+lastmod: '2021-01-18'
+tags: ['next-js', 'tailwind', 'guide']
+draft: false
+summary: 'Looking for a performant, out of the box template, with all the best in web technology to support your blogging needs? Checkout the Tailwind Nextjs Starter Blog template.'
+images: ['/static/images/canada/mountains.jpg', '/static/images/canada/toronto.jpg']
+---
+```
+
+### Compose
 
 `scripts/compose.js` can be used to easily generate a post with pre-filled front matter.