Set README.md as content for hex_doc index

[idlehands]

What does this do on a high level?

This updates the docs config so that when navigating to hexdocs.pm/checker_cab the contents of the README will be presented first.

How was this change made?

Added docs config into mix.exs

Why is this change necessary?

I received feedback that it was confusing to navigate the docs on hexdocs.pm

How will this be verified?

• [ ] The PR includes new tests
• [ ] Current test suite covers functionality
• [ ] This has been tested locally
• [ ] This has been tested in dev

Any additional information or special handling required on this PR?

motivational imagery

[coveralls-official[bot]]

Pull Request Test Coverage Report for Build 71ae2031e9bc1e5b4c45bb86527a9483f048cadd-PR-41

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 32f0a532b7241287509bf3c7d6f76268b767b1f7:	0.0%
Covered Lines:	43
Relevant Lines:	43

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💛 - Coveralls