Add OptiSim usage example to README

[mohini-aggarwal]

Adds a simple OptiSim usage example to the README.

• Demonstrates basic usage with a synthetic dataset
• Includes reproducibility via random seed
• Helps new users understand how to use select_from_cluster

[Copilot]

Pull request overview

This PR adds a minimal, reproducible README example demonstrating how to use the OptiSim distance-based subset selection method (select_from_cluster) on a synthetic dataset to help new users get started.

Changes:

• Add an “Example” section showing how to generate a small synthetic dataset with NumPy.
• Demonstrate initializing OptiSim and calling select_from_cluster to obtain a subset of indices.
• Print selected indices and the resulting selection size.

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[mohini-aggarwal]

Thanks, I have updated the example to clarify reproducibility and that sample size is approximate due to convergence.
PR is now ready for review.