docs(README): add note about expiration (actions#141)

addresses actions#121 (comment)

Co-authored-by: Parker Brown <[email protected]>

Author: gr2m

README.md

@@ -12,6 +12,9 @@ In order to use this action, you need to:
 2. [Store the App's ID in your repository environment variables](https://docs.github.com/actions/learn-github-actions/variables#defining-configuration-variables-for-multiple-workflows) (example: `APP_ID`)
 3. [Store the App's private key in your repository secrets](https://docs.github.com/actions/security-guides/encrypted-secrets?tool=webui#creating-encrypted-secrets-for-a-repository) (example: `PRIVATE_KEY`)
 
+> [!IMPORTANT]  
+> An installation access token expires after 1 hour. Please [see this comment](https://github.com/actions/create-github-app-token/issues/121#issuecomment-2043214796) for alternative approaches if you have long-running processes.
+
 ### Create a token for the current repository
 
 ```yaml