docs: Add AI Contribution policy to README

[sarina]

Elevating the AI Policy as defined in openedx/.github#223

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.29%. Comparing base (a995ce0) to head (177bb73).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1907   +/-   ##
=======================================
  Coverage   91.29%   91.29%           
=======================================
  Files         343      343           
  Lines        5770     5770           
  Branches     1388     1388           
=======================================
  Hits         5268     5268           
  Misses        483      483           
  Partials       19       19           

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