Update netutil pakcage

* Relocate `netutil` example
* Add README.md to run the example

src/examples/netutil/README.md

@@ -0,0 +1,18 @@
+## `netutil` example
+
+Thia is an example of `netutil` package.
+(see [netutil package](https://github.com/cloud-barista/cb-tumblebug/blob/main/src/core/common/netutil/netutil.go))
+
+### How to execute this example
+
+**Move to the example directory**
+
+```bash
+cd PROJECT_ROOT_DIR/src/examples/netutil
+```
+
+**Execute the example**
+
+```bash
+go run .
+```