Add performance section to float8 README.md

[vkuzo]

Adds a section to the float8 readme with context on how large of a M, K, N do you need to get a speedup.

#734 is needed to generate the table in this new section

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/794

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✅ You can merge normally! (2 Unrelated Failures)

As of commit bc41160 with merge base 65f660d ():

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Run Regression Tests / test (CPU Nightly, linux.4xlarge, --pre torch --index-url https://download.pytorch.org/whl/nightl... / linux-job (gh) (trunk failure)
• Run Regression Tests / test (CUDA Nightly, linux.g5.12xlarge.nvidia.gpu, --pre torch --index-url https://download.pytorc... / linux-job (gh) (trunk failure)

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