README: add a description about record_modifier
README: add a description about record_modifier

Signed-off-by: Takahiro YAMASHITA <8b9f03f78cb27351a99f79113f99e589b65190d5@gmail.com>

fluent/fluent-bit,takahasi/fluent-bit,fluent/fluent-bit,takahasi/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit,takahasi/fluent-bit,nokute78/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,takahasi/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit,takahasi/fluent-bit,nokute78/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit,takahasi/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit,takahasi/fluent-bit,nokute78/fluent-bit,fluent/fluent-bit,takahasi/fluent-bit