added stopwatch

docs/examples/stop-watch.md

@@ -0,0 +1,30 @@
+# Stop watch
+
+Press ``A`` to start the counter and 
+
+```blocks
+let msec = 0
+let sec = 0
+let end = 0
+let d = 0
+let start = 0
+input.onButtonPressed(Button.A, () => {
+    if (!(start)) {
+        start = input.runningTime()
+        end = 0
+    } else {
+        d = input.runningTime() - start
+        start = 0
+        basic.clearScreen()
+        basic.pause(1000)
+        sec = d / 1000
+        msec = d % 1000
+        basic.showString("" + sec + "." + msec)
+    }
+})
+basic.forever(() => {
+    if (start) {
+        led.toggle(Math.random(5), Math.random(5))
+    }
+})
+```