add a warning about usage preset-env and runtime together

README.md

@@ -280,6 +280,8 @@ Promise.resolve(32).then(x => console.log(x));
 
 By default, `@babel/runtime` only polyfills stable features, but like in `@babel/preset-env`, you can enable polyfilling of proposals by `proposals` option, as `corejs: { version: 3, proposals: true }`.
 
+**Warning!** If you use `@babel/preset-env` and `@babel/runtime` together, use `corejs` option only in one place since it's duplicate functionality and will cause conflicts.
+
 ### Configurable level of aggressiveness
 
 By default, `core-js` sets polyfills only when they are required. That means that `core-js` checks if a feature is available and works correctly or not and if it has no problems, `core-js` use native implementation.