Add elementary OS Juno (#1622)

Via https://github.com/nodesource/distributions.

Author: cassidyjames

locale/en/download/package-manager.md

@@ -111,6 +111,7 @@ The Node.js modules available in the distribution official repositories do not n
 * **elementary OS Luna** (via Ubuntu 12.04 LTS)
 * **elementary OS Freya** (via Ubuntu 14.04 LTS)
 * **elementary OS Loki** (via Ubuntu 16.04 LTS)
+* **elementary OS Juno** (via Ubuntu 18.04 LTS)
 
 **Supported Trisquel versions:**