dumpling: add a note about compression (#6619) (#7927)

dumpling-overview.md

@@ -253,6 +253,7 @@ The exported file is stored in the `./export-<current local time>` directory by
 
 - The `-t` option specifies the number of threads for the export. Increasing the number of threads improves the concurrency of Dumpling and the export speed, and also increases the database's memory consumption. Therefore, it is not recommended to set the number too large.
 - The `-r` option specifies the maximum number of records (or the number of rows in the database) for a single file. When it is enabled, Dumpling enables concurrency in the table to improve the speed of exporting large tables.
+- The `--compress gzip` option can be used to compress the dump. This can help to speed up dumping of data if storage is the bottleneck or if storage capacity is a concern. The drawback of this is an increase in CPU usage. Each file is compressed individually.
 
 With the above options specified, Dumpling can have a quicker speed of data export.