Improve Element example in README

[penberg]

This improves the Element example in README by adding pointers for get_child()
and get_attribute().

[flosse]

thx :)

[penberg]

@Florob ping

[Florob]

pong.
Sorry, not ignoring you I was just preoccupied with FrOSCon. Thank you for the patch.
I'm not quite happy with this though. Blindly unwraping Options is an anti-pattern IMHO, and I try to avoid it, even in documentation. If you'd instead use Option::and_then() and Option::expect(), to get to the href, or make href and Option<&str>, I'll gladly merge this.