docs: update README.md

[dominique-roux]

The log level for warning is WARN, not WARNING
If you use WARNING it is ignored.

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> ☕️

[google-cla]

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[codecov]

Codecov Report

Merging #174 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #174   +/-   ##
=========================================
  Coverage     68.05%   68.05%           
  Complexity       35       35           
=========================================
  Files             2        2           
  Lines           144      144           
  Branches         15       15           
=========================================
  Hits             98       98           
  Misses           33       33           
  Partials         13       13           

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