Add pinning note to readme

Author: thespad

readme-vars.yml

@@ -61,6 +61,8 @@ app_setup_block: |
 
   Formally only mongodb 3.6 through 4.4 are supported, however, it has been reported that newer versions will work. If you choose to use a newer version be aware that you will not be operating a supported configuration.
 
+  **Make sure you pin your database image version and do not use `latest`, as mongodb does not support automatic upgrades between major versions.**
+
   If you are using the [official mongodb container](https://hub.docker.com/_/mongo/), you can create your databases using an `init-mongo.js` file with the following contents:
 
   ```js