Add CODEOWNERS file (#40)

Does exactly what it says on the tin. Similar to some of our other open
source projects:

-
[CargoSense/eslint-config](https://github.com/CargoSense/eslint-config/blob/main/.github/CODEOWNERS)
-
[CargoSense/rubocop-cargosense](https://github.com/CargoSense/rubocop-cargosense/blob/main/.github/CODEOWNERS)
-
[CargoSense/stylelint-config-scss](https://github.com/CargoSense/stylelint-config-scss/blob/main/.github/CODEOWNERS)

.github/CODEOWNERS

@@ -0,0 +1 @@
+* @CargoSense/elixir-reviewers