docs: Liquid is not rendered in front matter

props: @beep @ashmaroli

docs/_docs/front-matter.md

@@ -20,7 +20,7 @@ Between these triple-dashed lines, you can set predefined variables (see below
 for a reference) or even create custom ones of your own. These variables will
 then be available for you to access using Liquid tags both further down in the
 file and also in any layouts or includes that the page or post in question
-relies on.
+relies on. 
 
 <div class="note warning">
   <h5>UTF-8 Character Encoding Warning</h5>
@@ -137,6 +137,30 @@ food: Pizza
 ```
 {% endraw %}
 
+
+Liquid does _not_ render when embedded in front matter. 
+
+For example if you have a liquid tag in a front matter value:
+
+{% raw %}
+```liquid
+---
+title: Post title 
+intro: This is a follow up of [this other post]{% link _posts/2020-01-01-previously.md %}).
+---
+```
+{% endraw %}
+
+You need to `capture` the Liquid expression first, before rendering it to Markdown.
+
+{% raw %}
+```liquid
+
+{% capture intro %}{{ page.info }}{% endcapture %}
+{{ intro | markdownify }}
+```
+{% endraw %}
+
 ## Predefined Variables for Posts
 
 These are available out-of-the-box to be used in the front matter for a post.