Document URLFWD record type

[patcon]

re-ticketed from: #52

This isn't well-documented in OctoDNS, but there's an redirect pseudo-type (not a real DNS record type) specifically for cloudflare. We should mention that there are only 3 rules available for free. And that redirecting "www" as well will consume a rule.

Code:
https://github.com/octodns/octodns-cloudflare/blob/dcb228e38fbc20e680a6eefa51068eebac201de5/octodns_cloudflare/__init__.py#L343-L354