docs: Update README.md with comprehensive features and examples

[john-aigora]

The README.md has been significantly updated to reflect the current capabilities of the sensPy library.

Key changes:

• Revised the introduction to state active development and broader feature set.
• Replaced the minimal 'Usage' section with an extensive 'Features' section.
• The 'Features' section now details:
  • Core Psychometric Models: BetaBinomial, discrim, twoAC, dod, samediff.
  • d-prime Hypothesis Testing: dprime_test, dprime_compare.
  • ROC Utilities: SDT, AUC.
  • Psychometric Link Functions: psyfun, psyinv, psyderiv, rescale.
  • Power and Sample Size: power_discrim, sample_size_for_binomial_power.
• Each feature includes a brief description, its sensR equivalent, and a concise, runnable Python code example.
• Ensured examples correctly import and use numpy where necessary.
• Retained existing 'Installation', 'Contributing', and 'License' sections.

This update provides a much clearer and more comprehensive overview for you and potential contributors.

Summary

Describe the changes in this pull request.

Testing

• pytest