Add note about extensions needing to be OSS (#113)

* Add note about extensions needing to be OSS

* Update README.md

* Update README.md

Author: Hayden Young

README.md

@@ -62,6 +62,10 @@ How to [secure your setup](/doc/security/ssl.md).
 - Electron and Chrome OS applications to bridge the gap between local<->remote.
 - Run VS Code unit tests against our builds to ensure features work as expected.
 
+### Notes
+
+- At the moment we can't use the official VSCode Marketplace. We've created a custom extension marketplace focused around open-sourced extensions. However, if you have access to the `.vsix` file, you can manually install the extension.
+
 ## Contributing
 
 Development guides are coming soon.