Document in-portal branch's ruleset customizations in README

[aik099]

Summary

• Adds a "Branch-specific Customizations" section to README.md explaining the ruleset.xml differences between this branch and master: path exclusions, disabled /core/ scope-indent check, and disabled missing-namespace check.

Test plan

• Docs-only change, no tests needed.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.72%. Comparing base (093e923) to head (dce3440).

Additional details and impacted files

@@             Coverage Diff              @@
##             in-portal     #118   +/-   ##
============================================
  Coverage        97.72%   97.72%           
  Complexity         513      513           
============================================
  Files               30       30           
  Lines             1493     1493           
============================================
  Hits              1459     1459           
  Misses              34       34           

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