Mention DCO as a requirement in CONTRIBUTORS.md

[adityasaky]

Summary

Add DCO as a requirement in the CONTRIBUTORS.md file.

Release Note

NONE

Documentation

NONE

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.91%. Comparing base (488eb97) to head (bb67f89).
Report is 219 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2255       +/-   ##
===========================================
- Coverage   66.46%   49.91%   -16.56%     
===========================================
  Files          92      192      +100     
  Lines        9258    24676    +15418     
===========================================
+ Hits         6153    12316     +6163     
- Misses       2359    11269     +8910     
- Partials      746     1091      +345     

Flag	Coverage Δ
e2etests	46.53% <ø> (-1.03%)	⬇️
unittests	41.92% <ø> (-5.76%)	⬇️

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[cpanato]

thanks