mx_formats: Update README.md

[vkuzo]

Clean up the MX readme for PTC.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/3210

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❌ 1 New Failure, 2 Unrelated Failures

As of commit a2a135d with merge base 94dee9c ():

NEW FAILURE - The following job has failed:

• Run Regression Tests (aarch64) / test-cpu-ops (macos-14) (gh)
  /Users/runner/miniconda3/envs/venv/lib/python3.10/site-packages/executorch/share/cmake/../../include/executorch/runtime/core/memory_allocator.h:143:21: error: no matching function for call to 'mul_overflows'

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 (aarch64) / test-cpu-ops (linux.arm64.2xlarge) (gh) (trunk failure)
  ##[error]The operation was canceled.
• Run Regression Tests / test-nightly (CUDA Nightly, linux.g5.12xlarge.nvidia.gpu, --pre torch --index-url https://downloa... / linux-job (gh) (trunk failure)
  test/dtypes/test_nf4.py::TestComm::test_comm

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