docs: Add Robinhood as user

[randxie]

Checklist:

• My organization is added to USERS.md.

[randxie]

cc: @terrytangyuan

[codecov]

Codecov Report

Merging #5589 (a3bdc83) into master (1dbae73) will decrease coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5589      +/-   ##
==========================================
- Coverage   47.10%   46.96%   -0.14%     
==========================================
  Files         240      240              
  Lines       15002    15002              
==========================================
- Hits         7066     7045      -21     
- Misses       7038     7060      +22     
+ Partials      898      897       -1     

Impacted Files	Coverage Δ
workflow/metrics/server.go	12.50% <0.00%> (-4.17%)	⬇️
server/workflow/workflow_server.go	38.74% <0.00%> (-3.99%)	⬇️
workflow/controller/operator.go	70.69% <0.00%> (-0.28%)	⬇️

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[terrytangyuan]

Thanks! LGTM.

[simster7]

Welcome! To the moon 🚀