Add CodeCov YAML file.

[MikeMcQuaid]

This should make the output and statuses a bit more friendly.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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[Bo98]

Is this in the right place?

the file must still be located in the root, dev/, or .github/ directories

[MikeMcQuaid]

@Bo98 This is where it was before because it's the root from where the tests are run. I'm not sure how you verify if it's the right place now, to be honest. Any ideas/help would be welcome.

[Bo98]

Well it seems to still be red when there is -0.01% changes and the links here: https://codecov.io/gh/Homebrew/brew/tree/master/cmd don't work.

[MikeMcQuaid]

#7895