doc(ember-simple-auth): start writing a migration guide

guides/upgrade-to-v7.md

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+### tl;dr
+
+Release 7.0.0 doesn't contain any new features and the API haven't really changed.
+It focuses on housekeeping, modernizing the codebase and introducing Typescript to our library.
+The biggest change was getting rid of as much "classic" Ember syntax as possible and get onto ES6 classes.
+
+### Extending ember-simple-auth's classes
+
+Most if not all of the public API is ES6 classes, as a result you'll have to change how ESA's classes are extended in your codebases.
+Here's an example based on the OAuth2PasswordGrant authenticator.
+
+Old
+
+```js
+import OAuth2PasswordGrant from 'ember-simple-auth/authenticators/oauth2-password-grant';
+import config from '../config/environment';
+
+export default OAuth2PasswordGrant.extend({
+  serverTokenEndpoint: `${config.apiHost}/token`,
+  serverTokenRevocationEndpoint: `${config.apiHost}/revoke`,
+});
+```
+
+New
+
+```js
+import OAuth2PasswordGrant from 'ember-simple-auth/authenticators/oauth2-password-grant';
+import config from '../config/environment';
+
+export default class OAuth2 extends OAuth2PasswordGrant {
+  serverTokenEndpoint = `${config.apiHost}/token`;
+  serverTokenRevocationEndpoint = `${config.apiHost}/revoke`;
+}
+```