Document addon installation in Karaf from Marketplace

as implemented in openhab/openhab-core#4057 .

configuration/addons.md

@@ -94,4 +94,14 @@ Place the .jar file in the folder Additional add-on files as described in File L
 ## Through the openHAB console
 
 Add-ons (also known as bundles) can be also installed and updated via the openHAB console.
-A detailed description can be found at [Installing or Updating Bundles](/docs/administration/bundles.html).
+
+To see the availabe add-ons in the market place:
+```sh
+openhab:addons list
+```
+To install an add-on in the background:
+```sh
+openhab:addons install <id-as-from-list-command>
+```
+
+Another approach can be found at [Installing or Updating Bundles](/docs/administration/bundles.html).