README.md in imap-backup-3.3.1 vs README.md in imap-backup-3.4.0

@@ -14,14 +14,22 @@
 [Source Code]: https://github.com/joeyates/imap-backup "Source code at GitHub"
 [API documentation]: http://rubydoc.info/gems/imap-backup/frames "RDoc API Documentation at Rubydoc.info"
 [Rubygem]: http://rubygems.org/gems/imap-backup "Ruby gem at rubygems.org"
 [Continuous Integration]: https://circleci.com/gh/joeyates/imap-backup "Build status by CirceCI"
 
-## GMail
+# GMail
 
-GMail OAuth2 authentication is supported.
+To use imap-backup with GMail, you will need to enable 'App passwords' on your account.
 
-To set it up, [follow the HOWTO](docs/setting-up-gmail.md).
+## GMail OAuth2
+
+GMail OAuth2 authentication is supported, but as GMail's policy requires
+users to set up an application specific to their account, the feature
+is disabled by default.
+
+You will need to set the environment variable IMAP_BACKUP_ENABLE_GMAIL_OAUTH2.
+
+To set it up, [follow the HOWTO](docs/setting-up-gmail-with-oauth2.md).
 
 # Installation
 
 ```shell
 $ gem install 'imap-backup'