Documentation for Sign In With Apple (#648)

* Documentation for Sign In With Apple

* fix markdown

* cleanup

Author: dplewis

_includes/parse-server/third-party-auth.md

@@ -2,6 +2,7 @@
 
 Parse Server supports 3rd party authentication with
 
+* Apple
 * Facebook
 * Facebook AccountKit
 * Github
@@ -119,6 +120,33 @@ Learn more about [Twitter login](https://developer.twitter.com/en/docs/twitter-f
 }
 ```
 
+### Apple `authData`
+
+As of Parse Server 3.5.0 you can use [Sign In With Apple](https://developer.apple.com/sign-in-with-apple/get-started/).
+
+```js
+{
+  "apple": {
+    "id": "jwt token for user", // required, used for validation
+    "access_token": "an authorized access token for the user", // optional
+  }
+}
+```
+
+#### Configuring parse-server for Sign In with Apple
+
+```js
+{
+  auth: {
+   apple: {
+     client_id: "", // optional (for extra validation), use the Service ID from Apple.
+   },
+  }
+}
+```
+
+Learn more about [Sign In With Apple](https://developer.okta.com/blog/2019/06/04/what-the-heck-is-sign-in-with-apple).
+
 ### Github `authData`
 
 ```js