add smart tools extension (#1814)

docs/extensions.md

@@ -187,6 +187,10 @@ Check out [microbit.org](http://microbit.org/resellers/) for more information on
     "name": "Pimoroni Envirobit",
     "url": "/pkg/pimoroni/pxt-envirobit",
     "cardType": "package"
+}, {
+    "name": "51bit Smart Tools",
+    "url": "/pkg/51bit/SmartTools",
+    "cardType": "package"
 }, {
     "name": "MakerBit",
     "url": "/pkg/1010Technologies/pxt-makerbit",