Information on Environment Variables

When playing with Phoenix @finnhodgkin and I have found the need to use Environment Variables, which we tussled a little bit with.

The `System` module allows us to get Environment Variables by keys, like so: `System.get_env("my_api_key")`

You can set the Environment Variables with the `source` bash command, but this has to be re-run in every new instance of a shell, which isn't ideal.

The [`Envy`](https://github.com/BlakeWilliams/envy) Elixir module solves this problem and was quite simple to set up (even for a Phoenix noob like myself).

I think a small section on how to set up Envy and use Environment Variables would be a good addition to this repo, since using Environment Variables to hide secret stuff is super important!

I'm super happy to make a PR doing so if the community think it would be a good addition!

Locate the repository's existing Phoenix setup or configuration documentation and review the shown System.get_env/Envy examples. Add a concise section explaining how to set up Envy and use environment variables for secrets, then verify that the guidance fits the surrounding documentation.

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