Add README documentation for scikit-learn MNIST example

[yeandy]

Add README for scikit-learn example

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Can one of the admins verify this patch?

[yeandy]

R: @AnandInguva

[yeandy]

R: @ryanthompson591@tvalentyn@rezarokni@pcoet

[codecov]

Codecov Report

Merging #21887 (6c5bd4f) into master (b1a313e) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #21887 +/- ##
==========================================
- Coverage 74.01% 74.01% -0.01% 
==========================================
Files 699 699 Lines 92675 92675 ==========================================
- Hits 68592 68591 -1 - Misses 22828 22829 +1 
Partials 1255 1255 

Flag	Coverage Δ
python	83.65% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdks/python/apache_beam/io/localfilesystem.py	90.97% <0.00%> (-0.76%)	⬇️
...eam/runners/portability/fn_api_runner/execution.py	92.44% <0.00%> (-0.65%)	⬇️
sdks/python/apache_beam/transforms/combiners.py	93.05% <0.00%> (-0.39%)	⬇️
...ks/python/apache_beam/runners/worker/sdk_worker.py	88.94% <0.00%> (-0.16%)	⬇️
sdks/python/apache_beam/transforms/util.py	96.06% <0.00%> (-0.16%)	⬇️
...examples/inference/pytorch_image_classification.py	0.00% <0.00%> (ø)
...eam/runners/interactive/interactive_environment.py	92.02% <0.00%> (+0.30%)	⬆️
...hon/apache_beam/runners/direct/test_stream_impl.py	94.02% <0.00%> (+0.74%)	⬆️
...che_beam/runners/interactive/interactive_runner.py	91.39% <0.00%> (+1.32%)	⬆️
...python/apache_beam/runners/worker/worker_status.py	79.71% <0.00%> (+1.44%)	⬆️
... and 1 more

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[AnandInguva]

LGTM! Thanks Andy.

Reference: Example PR #21781

[yeandy]

Unrelated (go licenses) failures.