Add warning about use of Mix.env/0 to README

[bitwalker]

Using Mix.env/0 in the configuration example is likely to lead to issues when someone tries to use code like that in a release, as Mix is not intended (and basically does not work) in releases. This PR adds a warning and alternative option for the same config, but it may be preferable to remove the use of Mix.env/0 entirely and use something else as an example.

Nice work!

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[coveralls]

Pull Request Test Coverage Report for Build 339

• 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 82.083%

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Totals
Change from base Build 337:	0.0%
Covered Lines:	197
Relevant Lines:	240

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💛 - Coveralls

[zachdaniel]

I think I agree that just entirely removing that example would be best. Perhaps setting it to some evaluation of a system variable, or something else to illustrate that it can be done dynamically. Feel free to add a better example to remove the need for the warning, or to just remove that section entirely and we can add an example afterwards.

Thanks for your contribution!

[bitwalker]

@zachdaniel Sure thing! I've updated the example to use System.get_env/1 instead, I think that covers the same purpose and doesn't carry the baggage of Mix.env/0.

[zachdaniel]

🎉

[GregMefford]

Thanks @bitwalker! ❤️