update OSS documentation with nnlib instructions

[zonglinpeng]

Summary: titled

Differential Revision: D96954586

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18245

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❌ 1 Cancelled Job, 4 Pending, 2 Unrelated Failures

As of commit 997a65f with merge base b7ca1a4 ():

CANCELLED JOB - The following job was cancelled. Please retry:

• periodic / gather-models (gh)

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

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

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[meta-codesync]

@zonglinpeng has exported this pull request. If you are a Meta employee, you can view the originating Diff in D96954586.

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