Amend README with how to configure shellcheck in addition to the pre-…

[tonyfruzza]

Description

Amends README.md with information on how to configure shellcheck beyond the --enable flag

resolves #79

Documentation

Updated README.md to describe that .shellcheckrc can be used in addition to the --enable flag.

TODOs

Please ensure all of these TODOs are completed before asking for a review.

• [x] Ensure the branch is named correctly with the issue number. e.g: feature/new-vpc-endpoints-955 or bug/missing-count-param-434.
• [x] Update the docs.
• [x] Keep the changes backward compatible where possible.
• [x] Run the pre-commit checks successfully.
• [x] Run the relevant tests successfully.
• [x] Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.

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