IE 11 incompatibility due to use of .includes() and .findIndex()

The docs describe using the intersection observer polyfill in order to support older browsers like IE, but they don't describe additional requirements to polyfill functions like `Array.prototype.includes` and `Array.prototype.findIndex`.

It'd be great if usage of those functions could be avoided or, at the very least, have their usage documented so that any poor souls supporting IE know what to polyfill.

If anyone else runs into this, if you're not auto-polyfilling using `babel-preset-env` then the following is sufficient as of `v1.0.0-rc.2`:

```js
import "core-js/modules/es.array.find-index";
import "core-js/modules/es.array.includes";
import "intersection-observer"
```