docs(upgrade): Add note regarding min Sentry 9.1.2 requirement (#380)

Fixes #325.

Author: BYK

README.md

@@ -39,6 +39,8 @@ and [Nginx](http://nginx.org/). You'll likely want to add this service to your `
 
 ## Updating Sentry
 
+_You need to be on at least Sentry 9.1.2 to be able to upgrade automatically to the latest version. If you are not, upgrade to 9.1.2 first by checking out the [9.1.2 tag](https://github.com/getsentry/onpremise/tree/9.1.2) on this repo._
+
 The included `install.sh` script is meant to be idempotent and to bring you to the latest version. What this means is you can and should run `install.sh` to upgrade to the latest version available. Remember that the output of the script will be stored in a log file, `sentry_install_log-<ISO_TIMESTAMP>.txt`, which you may share for diagnosis if anything goes wrong.
 
 ## Resources