docs: add meaningful change threshold guide

[DevALVIN-24]

Closes #502

Summary

This PR introduces the Meaningful Change Threshold Guide to ensure that contributions prioritize quality, completeness, and test coverage over mere line counts.

Changes Made

• New Guide (docs/meaningful-change-threshold-guide.md):
  • Explains that line count is not the primary standard for judging a PR.
  • Differentiates between small but complete changes (meaningful) and small incomplete changes (insufficient).
  • Provides concrete examples of insufficient changes such as drive-by formatting and tests without implementations.
  • Includes reviewer assessment guidance focusing on acceptance criteria, test evidence, and overall quality.
• README Update: Linked the new guide in the README.md documentation section for easy access by contributors and reviewers.

Motivation

Some PRs previously submitted included very small code changes without fully solving the underlying issue. This guide sets a clear expectation that changes must be complete, tested, and aligned with the issue requirements, regardless of their size.