Add README documentation for ONNX import examples

[NA-V10]

This PR adds a new README.md file for the onnx-import-examples module.

✔ What’s included

• Overview of ONNX import functionality in DL4J
• Instructions on how to run the examples
• Folder structure explanation
• Notes on required ONNX model files
• Example expected output

✔ Why this is useful

The ONNX example folder previously had no README, which made it difficult for new users to understand how to run the examples or where to place model files.
This README improves documentation clarity and onboarding experience for beginners.

No code changes; documentation only.