Merge pull request #421 from h3nr1ke/h3nr1ke/apple-relay-information

Update README.md

packages/sign_in_with_apple/sign_in_with_apple/README.md

@@ -51,6 +51,11 @@ Your server should then daily verify the session with Apple (via a refresh token
 
 Before you can start integrating (or even testing) Sign in with Apple you need a [paid membership to the Apple Developer Program](https://developer.apple.com/programs/). Sign in with Apple is one of the restricted services which is not available for free with just an Apple ID ([source](https://developer.apple.com/programs/whats-included/)).
 
+#### Apple Mail Relay
+
+Since the users can use the private email relay, it is necessary to add an SPF record to the domains used to send emails, please read more here: https://developer.apple.com/help/account/configure-app-capabilities/configure-private-email-relay-service/
+Without it, your service will not be able to send emails to users who choose to use Apple's private relay, and the emails will not be delivered.
+
 ### Setup
 
 #### Register an App ID