gui: Add forgotten translation string

gui/default/assets/lang/lang-en.json

@@ -10,6 +10,7 @@
    "Add Device": "Add Device",
    "Add Folder": "Add Folder",
    "Add Remote Device": "Add Remote Device",
+   "Add devices from the introducer to our device list, for mutually shared folders.": "Add devices from the introducer to our device list, for mutually shared folders.",
    "Add new folder?": "Add new folder?",
    "Address": "Address",
    "Addresses": "Addresses",