Update README with ZenFlow release blog featured by PyTorch. (#7520)
**Main change:**
Add post bullet and link to ZenFlow release blog on latest news.
**Blog link:**
https://pytorch.org/blog/zenflow-stall-free-offloading-engine-for-llm-training/
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Signed-off-by: Tingfeng Lan <erc8gx@virginia.edu>