Add level 6

Author: gbaman

pxt.json

@@ -29,7 +29,8 @@
       "tutorials/cyber-kingdom/firewall/firewall_2.md",
       "tutorials/cyber-kingdom/firewall/firewall_3.md",
       "tutorials/cyber-kingdom/firewall/firewall_4.md",
-      "tutorials/cyber-kingdom/firewall/firewall_5.md"
+      "tutorials/cyber-kingdom/firewall/firewall_5.md",
+      "tutorials/cyber-kingdom/firewall/firewall_6.md"
     ],
     "testFiles": [
     ],