add note about FP6 kernel

torchao/csrc/fp6_llm/README.md

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+# FP6-LLM kernel
+
+This kernel is adapted from https://github.com/usyd-fsalab/fp6_llm. It performs linear op (A @ W.T), where A is in FP16 and W is in FP6 (E3M2 without infinities and NaN).
+
+On most hardware, this kernel is faster than FP16 linear for batch size from 1 to 128, and slower for batch size larger than or equal to 256. See https://github.com/usyd-fsalab/fp6_llm/issues/8 for a detailed discussion.
+
+See https://github.com/pytorch/ao/pull/223 for some benchmark results.