Update CONTRIBUTING.md

Co-authored-by: Michael Smith <throwawayclover@gmail.com>

Author: bcpeinhardt

CONTRIBUTING.md

@@ -135,4 +135,4 @@ to make sure we're using up to date versions of the client.
 3. Push a tag matching the new package.json version.
 4. Update the resulting draft release with the changelog contents.
 5. Publish the draft release.
-6. Download the `.vsix` file from the release and upload to the marketplace (both Microsoft and OpenVSX).
+6. Download the `.vsix` file from the release and upload to both the [official VS Code Extension Marketplace](https://code.visualstudio.com/api/working-with-extensions/publishing-extension), and the [open-source VSX Registry](https://open-vsx.org/).