[AIRFLOW-XXX] Add Zynga to list of users

[cjsekl]

Make sure you have checked all steps below.

Jira

• N/A, README change

Description

• Adding Zynga to list of users in README

Tests

• N/A, README change

Commits

• [x]

Documentation

• N/A, README change

Code Quality

• N/A, README change

[codecov-io]

Codecov Report

Merging #5460 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5460   +/-   ##
=======================================
  Coverage   79.13%   79.13%           
=======================================
  Files         488      488           
  Lines       30608    30608           
=======================================
  Hits        24222    24222           
  Misses       6386     6386

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