Create env.md

docs/env.md

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+## ENV
+ENV is a class than enables to interact with environmental variables
+
+### Class methods
+```swift
+// read an env variable
+var value = ENV.get("VAR_NAME")
+
+// write an env value
+ENV.set("VAR_KEY", "VAR_VALUE")
+
+// list all env variables
+var list = ENV.keys()
+```