docs: add iTunes podcast feed example

[oiahoon]

Summary

Adds an executable RSS 2.0 example showing how to set iTunes channel and item elements with RSS::Maker.

Closes #71

Verification

• Executed and parsed the exact README example against the local library
• ruby test/run-test.rb (321 tests, 4,852 assertions)
• gem build rss.gemspec
• git diff --check

[kou]

Thanks.

@snex Could you try the example?

[snex]

Yes that works! Thank you very much!

[kou]

Thanks for confirming it.

I've merged this.