chore: Add sample into README.md about paths=source_relative

Author: mztnnrt

README.md

@@ -143,7 +143,7 @@ code.
 
 **Note**: by default **example.pb.validate.go** is nested in a directory
 structure that matches your `option go_package` name. You can change this using
-the protoc parameter `paths=source_relative:.`. Then `--validate_out` will
+the protoc parameter `paths=source_relative:.`, as like `--validate_out="lang=go,paths=source_relative:../generated"`. Then `--validate_out` will
 output the file where it is expected. See Google's protobuf documentation
 or [packages and input paths](https://github.com/golang/protobuf#packages-and-input-paths)
 or [parameters](https://github.com/golang/protobuf#parameters) for more