Add mindustry.io to servers.json (#1467)

- huge community (~1.5k members)
- anti-grief measures (discord bot <-> in-game moderation)
- huge map pool (50+)
- stable, powerful server based in europe

servers.json

@@ -10,5 +10,17 @@
   },
   {
     "address": "mindustry.ru"
+  },
+  {
+    "address": "mindustry.io"
+  },
+  {
+    "address": "mindustry.io:1000"
+  },
+  {
+    "address": "mindustry.io:2000"
+  },
+  {
+    "address": "mindustry.io:3000"
   }
 ]