Document new contributor YAML frontmatter

Author: swinton

content/README.md

@@ -25,6 +25,7 @@ See the [contributing docs](/CONTRIBUTING.md) for general information about work
   - [`includeGuides`](#includeGuides)
   - [`type`](#type)
   - [`topics`](#topics)
+  - [`contributor`](#contributor)
   - [Escaping single quotes](#escaping-single-quotes)
 - [Autogenerated mini TOCs](#autogenerated-mini-tocs)
 - [Versioning](#versioning)
@@ -231,6 +232,18 @@ includeGuides:
 - Type: `String`
 - Optional.
 
+### `contributor`
+- Purpose: Indicate an article is contributed and maintained by a third-party organization, typically a GitHub Technology Partner.
+- Type: `Object`. Properties are `name` and `URL`.
+- Optional.
+
+Example:
+
+```yml
+contributor:
+  name: ACME, inc.
+  URL: https://acme.example.com/
+```
 
 ### Escaping single quotes