Add winter.user.activate event to README (#20)

Fired here:
https://github.com/wintercms/wn-user-plugin/blob/010c32abdfde761287e3e78169ec1f4cc5933eb1/models/User.php#L104

README.md

@@ -303,6 +303,7 @@ This plugin will fire some global events that can be useful for interacting with
 - **winter.user.beforeAuthenticate**: Before the user is attempting to authenticate using the Account component.
 - **winter.user.login**: The user has successfully signed in.
 - **winter.user.logout**: The user has successfully signed out.
+- **winter.user.activate**: The user has activated their own account by email validation.
 - **winter.user.deactivate**: The user has opted-out of the site by deactivating their account. This should be used to disable any content the user may want removed.
 - **winter.user.reactivate**: The user has reactivated their own account by signing back in. This should revive the users content on the site.
 - **winter.user.getNotificationVars**: Fires when sending a user notification to enable passing more variables to the email templates. Passes the `$user` model the template will be for.