docs: Add Riskified to the user list

[OmerKahani]

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I have written unit and/or e2e tests for my change. PRs without these are unlike to be merged.
• Optional. I've added My organization is added to the USERS.md.
• I've signed the CLA and required builds are green.

[simster7]

Welcome to Argo!

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@8b92d33). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #2558   +/-   ##
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  Coverage          ?   11.21%           
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  Files             ?       75           
  Lines             ?    31838           
  Branches          ?        0           
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  Hits              ?     3571           
  Misses            ?    27783           
  Partials          ?      484

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