README: add feature and limitation

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Author: daipom

README.md

@@ -243,6 +243,58 @@ Especially, if the `in_tail` uses [follow_inodes](https://docs.fluentd.org/input
 
 Please consider updating Fluentd if you are using any version older than Fluentd v1.16.2 (td-agent v4.5.1).
 
+## Feature
+
+`tailcheck` performs the following checks on the specified pos file.
+
+* duplication check
+* collection ratio check
+
+### Duplication check
+
+`tailcheck` checks whether there is any dulication in the specified pos files.
+
+The keys of watching pos entries in one pos file must be unique.
+
+By default, the key is the path of the target log file.
+When using [follow_inodes](https://docs.fluentd.org/input/tail#follow_inodes), the key is the inode.
+
+If duplication occurs, it means some anomalies occurs in that `in_tail`.
+Especially, the following log missing issue causes this duplication.
+
+> * https://github.com/fluent/fluentd/issues/3614
+>   * In case `follow_inodes false` (default setting), collection of a file may stop and continue to stop after log rotation.
+>   * Fixed since Fluentd v1.16.3 (fluent-package v5.0.2, td-agent v4.5.2).
+
+### Collection ratio check
+
+`tailcheck` checks collection ratio of each watching pos entry.
+
+By default, `tailcheck` detects log files that have not been collected up to 50% of the filesize.
+You can change this threshold by `--ratio` option.
+Please see `Usage` section for details.
+
+If this it too low, collection of those files may not be keeping up.
+Or it may have stopped with some anomalies.
+
+Especially, if the `in_tail` uses [follow_inodes](https://docs.fluentd.org/input/tail#follow_inodes), there is a known log missing issue that was fixed in Fluentd v1.16.2.
+
+> * https://github.com/fluent/fluentd/issues/4190
+>   * In case `follow_inodes true`, collection of a file may stop and continue to stop after log rotation.
+>   * Fixed since Fluentd v1.16.2 (fluent-package v5.0.0, td-agent v4.5.1).
+
+#### Limitation
+
+`tailcheck` checks the sizes of logfiles based on the paths recorded in the pos files (except already unwatched pos entries).
+
+However, if using `--follow_inodes`, there is a restriction on which files can be checked.
+If using `--follow_inodes`, it is possible that `tailcheck` can not check the already rotated logfiles even if they are recorded in the pos files and are not unwatched yet.
+Since the path recorded in the pos file is not updated after log rotation, the current path and inode may differ.
+`tailcheck` checks only log files whose path and inode in the pos file match.
+At least, it can check the current log files.
+
+If not using `--follow_inodes`, this limitation does not exist.
+
 ## Development
 
 ### Quick start