Update Magento_AuthorizenetCardinal ReadMe

[diazwatson]

Description (*)

Updates ReadMe for the Magento_AuthorizenetCardinal module.

Fixed Issues (if relevant)

1. magento/devdocs README content for modules - work in M2 Code devdocs#4451: README content for modules - work in M2 Code

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds are green)

[m2-assistant]

Hi @diazwatson. Thank you for your contribution
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[magento-engcom-team]

Hi @jeff-matthews, thank you for the review.
ENGCOM-5725 has been created to process this Pull Request

[magento-engcom-team]

Hi @rogyar, thank you for the review.
ENGCOM-5725 has been created to process this Pull Request

[m2-assistant]

Hi @diazwatson, thank you for your contribution!
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