Add an example to illustate populating and traveling a subtree

[ZekeLu]

We understand why node.Children is no populated most of the time, but sometimes users get confused. Here are some related issues:

• Child nodes are not loaded using Nodes() chromedp#332
• Getting Children of a specific node is impossible chromedp#632
• how to retrieve DOM tree,including entire subtree? chromedp#692
• ChildNodeCount larger than 0 but Children slice is empty chromedp#761

So here comes the example.

I know that this practice should not be encouraged. So it's okay to refuse this pull request.

[kenshaw]

Merged.