Add upgrading to v8 guide

UPGRADING.md

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+# Upgrading
+
+## v8
+
+The most notable breaking changes introduced in v8 are:
+
+1. The `includePolyfill` option is no longer supported ([458](https://github.com/babel/ember-cli-babel/pull/458))
+2. `@babel/core` is now a required peer dependency ([452](https://github.com/babel/ember-cli-babel/pull/452))
+
+### Upgrade Path for Apps
+
+Apps that are relying on the `includePolyfill` option should install `core-js`, 
+and import `core-js/stable` directly in `app.js`. The reason for this is that, 
+`@babel/polyfill` has been deprecated. More info on this deprecation can be 
+found in [the documentation for `@babel/polyfill`](https://babeljs.io/docs/en/babel-polyfill).
+
+Apps are now also required to install `@babel/core` directly in order to use 
+`ember-cli-babel`. Making `@babel/core` a peer dependency ensures that the 
+same version is used by all tooling that require it.
+
+### Upgrade Path for Addons
+
+Since (v1) addons bring in their own version of `ember-cli-babel`, they should 
+now also bring in their own version of `@babel/core`. This means that, addons 
+should add `@babel/core` under `dependencies` in their `package.json` file. 
+This makes the dependency on `@babel/core` more explicit while also avoiding 
+addons having to cut a breaking release to update `ember-cli-babel` to v8.