change error level from Fluent::ConfigError to $log.warn for detecting jump of rewriterule index

This change gets supporting jumped index number of rewriterules like below.
<match foo.bar>
  type rewrite_tag_filter
  rewriterule1 domain ^www\.google\.com$ site.Google
  rewriterule20 domain ^news\.google\.com$ site.GoogleNews
</match>

Thank you for reporting by m.yuzuki (@ephemeralsnow)
https://twitter.com/ephemeralsnow/status/349039629967716352
update README