[#172] Update README.md

Add info about `xk6-registry import` to README.md.

Signed-off-by: Ekaterina Lebedeva <ekaterina.lebedeva@yadro.com>

README.md

@@ -186,6 +186,25 @@ Flags:
 -v, --version         version for registry-exporter
 ```
 
+## Import pregen into registry db
+
+You can import pregenerated json files into registry bolt db. Use `frostfs-xk6-registry import`. Usage examples are in help:
+
+```shell
+$ ./bin/frostfs-xk6-registry import -h
+Import objects into registry from pregenerated files
+
+Usage:
+  xk6-registry import [flags]
+
+Examples:
+xk6-registry import registry.bolt preset.json
+xk6-registry import registry.bolt preset.json another_preset.json
+
+Flags:
+  -h, --help   help for import
+```
+
 # License
 
 - [GNU General Public License v3.0](LICENSE)