adding turtle examples (#598)

docs/examples/turtle-spiral.md

@@ -0,0 +1,23 @@
+# Turtle Spiral
+
+The turtle goes to the center back and forth.
+
+```blocks
+let index = 0
+turtle.setPosition(0, 0)
+turtle.turnRight()
+basic.forever(() => {
+    for (let index = 0; index <= 4; index++) {
+        turtle.forward(4 - index)
+        turtle.turnRight()
+    }
+    for (let index = 0; index <= 4; index++) {
+        turtle.turnLeft()
+        turtle.back(index)
+    }
+})
+```
+
+```package
+microturtle=github:Microsoft/pxt-microturtle#master
+```