add a note about BigInt and JSBI, close #381

README.md

@@ -2852,7 +2852,8 @@ console.log(getIteratorMethod({})); // undefined
 ```
 
 ## Missing polyfills[⬆](#index)
+- ES `BigInt` can't be polyfilled since it requires changes in the behavior of operators, you could find more info [here](https://github.com/zloirock/core-js/issues/381). You could try to use [`JSBI`](https://github.com/GoogleChromeLabs/jsbi).
+- ES `Proxy` can't be polyfilled, you can try to use [`proxy-polyfill`](https://github.com/GoogleChrome/proxy-polyfill) which provides a very little subset of features.
 - ES `String#normalize` is not a very useful feature, but this polyfill will be very large. If you need it, you can use [unorm](https://github.com/walling/unorm/).
-- ES `Proxy` can't be polyfilled, you can try [`proxy-polyfill`](https://github.com/GoogleChrome/proxy-polyfill) which provides a very little subset of features.
-- `window.fetch` is not a cross-platform feature, in some environments, it makes no sense. For this reason, I don't think it should be in `core-js`. Looking at a large number of requests it *might be*  added in the future. Now you can use, for example, [this polyfill](https://github.com/github/fetch).
 - ECMA-402 `Intl` is missed because of the size. You can use [those polyfills](https://formatjs.io/docs/polyfills).
+- `window.fetch` is not a cross-platform feature, in some environments, it makes no sense. For this reason, I don't think it should be in `core-js`. Looking at a large number of requests it *might be*  added in the future. Now you can use, for example, [this polyfill](https://github.com/github/fetch).