Explain gem pinning strategy

Peter Leitzenrequested to merge
doc/gem-pinning-strategy into master

Summary

This MR documents why gitlab-styles pins specific versions of RuboCop and related gems.

Changes

Adds a new "Why We Pin Gem Versions" section to the README that explains:

  • Rationale: Ensures consistent rule configuration and behavior across all GitLab projects
  • Benefits: Prevents unexpected rule changes, inconsistent linting results, and configuration drift

Related Issues

Closes #N/A (documentation improvement)

Edited by Peter Leitzen

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