examples: re-add hello world serial communication

I accidentally removed the serial example from 45348bfc3e while merging
commit 40f834d58f (PR: https://github.com/aykevl/tinygo/pull/12).
Force-pushing seemed like a bad idea so here it is.

src/examples/serial/serial.go

@@ -0,0 +1,10 @@
+package main
+
+import "time"
+
+func main() {
+	for {
+		println("hello world!")
+		time.Sleep(time.Second)
+	}
+}