Add American Express to list of users on readme

[TheLastSultan]

Documentation

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• Bug Fix
• Enhancement (new features, refinement)
• Refactor
• Add tests
• Build / Development Environment
• [x ] Documentation

SUMMARY

Add American Express to list of users on readme.md

REVIEWERS

[codecov-io]

Codecov Report

Merging #7576 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7576   +/-   ##
=======================================
  Coverage   65.17%   65.17%           
=======================================
  Files         433      433           
  Lines       21428    21428           
  Branches     2360     2360           
=======================================
  Hits        13966    13966           
  Misses       7342     7342           
  Partials      120      120

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[mistercrunch]

Welcome aboard!