Add StemyPlume to the instance list

As requested here: https://github.com/Plume-org/Plume/issues/814

source/javascripts/instances.json

@@ -70,6 +70,11 @@
       "name": "Plume Luciférienne",
       "description": "Instance personnelle mais ouverte à tou·tes",
       "url": "https://plume.luciferi.st"
+    },
+    {
+      "name": "StemyPlume",
+      "description": "Instance pour mon usage personnel, mais accessible à qui veut.",
+      "url": "https://plume.stemy.me"
     }
   ]
 }