adding turtle examples (#598)

docs/examples/turtle-square.md

@@ -0,0 +1,17 @@
+# Turtle Square
+
+A turtle program that runs in a square.
+
+```blocks
+turtle.setPosition(0, 0)
+turtle.turnRight()
+turtle.setSpeed(29)
+basic.forever(() => {
+    turtle.forward(4)
+    turtle.turnRight()
+})
+```
+
+```package
+microturtle=github:Microsoft/pxt-microturtle#master
+```