Document requirements for iframe embedding for Chromecast receiver

[joeyparrish]

Sites can prevent iframe-embedding with the X-Frame-Options header. Though this should not be an issue for a test runner, one of the first things a person might try with the command-line tools is:

chromecast-webdriver-cli --hostname=chromecast-hostname --url=https://www.google.com/

However, google.com prevents iframe embedding, which means it cannot be shown in our Chromecast receiver. The user would see a blank screen instead. This should be clearly documented to avoid confusion.

[joeyparrish]

And in fact, we use www.google.com as an example in README.md, in spite of the fact that Google Search blocks iframe embedding. This was a particularly bad choice for the docs of this backend.