Add about partial_key, partial_value, and keep_partial_key
Add about partial_key, partial_value, and keep_partial_key

Signed-off-by: Kenji Okimoto <7a4b90a0a1e6fc688adec907898b6822ce215e6c@clear-code.com>

fluent-plugins-nursery/fluent-plugin-concat,okkez/fluent-plugin-concat