Update README.md

Author: Antoine Kevin

README.md

@@ -38,6 +38,7 @@ use Intercom\IntercomClient;
 
 $client = new IntercomClient(appId, apiKey);
 ```
+> Warning: API Keys are being deprecated - you should use a [personal access token](https://app.intercom.io/a/apps/_/settings/personal-access-token) instead. [Learn more](https://developers.intercom.io/docs/personal-access-tokens) about API Keys deprecation
 
 Or if using an OAuth or Personal Access Token use: