include additional documentation as a valid extension.

docs/manuals/marketplace/packages.md

@@ -122,17 +122,22 @@ publishing a public Marketplace listing page.
 
 Users may also provide their additive package content, which renders on the listing page for that package version.
 
-The Marketplace currently supports optionally adding an icon, release notes, readme, and SBOMs. By convention, organize these files in a directory-of-directories hierarchy,
-typically in the source tree. For example, the following is a valid layout for an icon, readme, and release notes:
+The Marketplace currently supports optionally adding an icon, release notes, readme, additional documentation, and SBOMs.
+By convention, organize these files in a directory-of-directories hierarchy,
+typically in the source tree. For example, the following is a valid layout for each supported extension:
 
 ```text
 extensions
+├── docs
+│   └── developing.md
 ├── icons
 │   └── icon.svg
 ├── readme
 │   └── readme.md
 └── release-notes
-    └── release_notes.md
+│   └── release_notes.md
+├── sbom
+│   └── sbom.spdx.json
 ```
 
 Then, to add these assets to your package version, use the `up` CLI:
@@ -142,9 +147,9 @@ up alpha xpkg append --extensions-root=./extensions xpkg.upbound.io/<your accoun
 ```
 
 :::warning
-This is an alpha feature, and requires `up` CLI version `v0.39.0` or later. See
+`xpkg append` an alpha feature, and requires `up` CLI version `v0.39.0` or later. See
 [`up alpha xpkg append`][up-alpha-xpkg-append] for complete
-documentation and conventions. 
+documentation and conventions.
 :::
 
 ## Troubleshooting