[DT-295] Add documentation for eslint config

[fboulnois]

Addresses

https://broadworkbench.atlassian.net/browse/DT-295

Summary

This clarifies why the eslint config is the way it is.

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Have you read Terra's Contributing Guide lately? If not, do that first.

• Label PR with a Jira ticket number and include a link to the ticket
• Label PR with a security risk modifier [no, low, medium, high]
• PR describes scope of changes
• Get a minimum of one thumbs worth of review, preferably two if enough team members are available
• Get PO sign-off for all non-trivial UI or workflow changes
• Verify all tests go green
• Test this change deployed correctly and works on dev environment after deployment

[rushtong]

👍🏽