Update README.md

peak#529
add suggestion on sync from between cloud

README.md

@@ -400,6 +400,14 @@ cp s3://bucket/prefix/index.html index.html
 cp s3://bucket/prefix/test.html test.html
 ```
 
+We don't support syncing between 2 storage endpoints out of the box. You may use s5cmd sync for this:
+
+```
+s5cmd sync 's3://s3-bucket/path/*' download_folder/
+
+s5cmd --endpoint-url <gcs-endpoint> sync 'download_folder/*' s3://gcs-bucket/path/
+```
+
 ##### Strategy
 ###### Default
 By default `s5cmd` compares files' both size **and** modification times, treating source files as **source of truth**. Any difference in size or modification time would cause `s5cmd` to copy source object to destination.