Update Magento_Backend module ReadMe

[diazwatson]

Description (*)

Updates ReadMe for the Magento_Backend 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
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento give me test instance - deploy test instance based on PR changes
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For more details, please, review the Magento Contributor Guide documentation.

[magento-engcom-team]

Hi @jeff-matthews, thank you for the review.
ENGCOM-5846 has been created to process this Pull Request

[m2-assistant]

Hi @diazwatson, thank you for your contribution!
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