Add shim/README.md to explain what the shim/ tree is for

[dbort]

Summary

It's not obvious why this tree is here. Add a README explaining it.

Test plan

Previewed in github.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/6860

• 📄 Preview Python docs built from this PR

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As of commit 893c99c with merge base f01b20b ():
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[dbort]

Jobs like pull / test-models-linux (cmake, mv3, portable, linux.2xlarge, 90) / linux-job (pull_request) are succeeding, so it looks like this file doesn't confuse buck2.