Update README.md to announce that OpenVINO GenAI uses XGrammar (#413)

[OpenVINO GenAI](https://github.com/openvinotoolkit/openvino.genai)
2025.3.0 version adds Structured Output support with XGrammar backend.

[Release
notes](https://docs.openvino.ai/2025/about-openvino/release-notes-openvino.html)

Samples:

https://github.com/openvinotoolkit/openvino.genai/blob/master/samples/python/text_generation/structured_output_generation.py

https://github.com/openvinotoolkit/openvino.genai/blob/master/samples/python/text_generation/structural_tags_generation.py

https://github.com/openvinotoolkit/openvino.genai/blob/master/samples/python/text_generation/compound_grammar_generation.py

https://github.com/openvinotoolkit/openvino.genai/blob/master/samples/cpp/text_generation/structured_output_generation.cpp

Author: apaniukov

README.md

@@ -16,6 +16,7 @@
 </div>
 
 ## News
+- [2025/09] XGrammar has been officially integrated into [OpenVINO GenAI](https://github.com/openvinotoolkit/openvino.genai)
 - [2025/02] XGrammar has been officially integrated into [Modular's MAX](https://docs.modular.com/max/serve/structured-output)
 - [2025/01] XGrammar has been officially integrated into [TensorRT-LLM](https://github.com/NVIDIA/TensorRT-LLM).
 - [2024/12] XGrammar has been officially integrated into [vLLM](https://github.com/vllm-project/vllm).