Add node on environment variable names

CONTRIBUTING.md

@@ -11,6 +11,19 @@ We prioritize clear and easy-to-understand code over complex or "magical" soluti
 If you're considering making a code change that might be considered overcomplicated or not in line with the project's goals, 
 we recommend opening an issue to discuss the change before submitting a pull request.
 
+#### Note on environment variables names
+
+Cleanenv strives to be as explicit and as obvious as possible, and as such, 
+we decided to write environment variable in config struct tags *as is*.
+It was done for purpose. This decision guarantees that programmer can *always* find environment variable by its name in code.
+It may not sound pretty useful for such a limitation, but in my own experience, it is.
+So you always will be able to find environment variable by its whole name in code,
+which makes writing complex nested config structures a bit more difficult.
+
+By the same reason, any contribution changing this behavior will be rejected.
+Please not add any prefixes or suffixes to environment variables names, we will just not approve such PRs.
+Thanks for understanding.
+
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