Add documentation

contributing/content-markup-reference.md

@@ -62,9 +62,11 @@ Octicons are icons used across GitHub’s interface. We reference Octicons when
 
 ### Usage
 
-`{% octicon "<name of octicon>" %}`  
-`{% octicon "plus" %}`
-`{% octicon "plus" aria-label="The plus icon" %}`
+```
+{% octicon "<name of octicon>" %}
+{% octicon "plus" %}
+{% octicon "plus" aria-label="The plus icon" %}
+```
 
 ## Operating system tags
 
@@ -94,6 +96,16 @@ These instructions are pertinent to Windows users.
 {% endlinux %}
 ```
 
+The platform-specific content matching the reader's operating system shows by default. You can override this behavior for individual Markdown files with the `defaultPlatform` frontmatter (such as for pages about GitHub Actions runners, which mostly use Linux):
+
+```
+---
+title: …
+versions: …
+defaultPlatform: linux
+---
+```
+
 ## Reusable and variable strings of text
 
 Reusable strings (commonly called content references or conrefs) contain content that’s used in more than one place in our documentation and allow us to change the content in a single location rather than every place the string appears.