Update README.md to mention ISLANDORA_TAG (#49)

README.md

@@ -238,6 +238,14 @@ name. If you have multiple sites on the same host, these must be unique.
 COMPOSE_PROJECT_NAME=isle-site-template
 ```
 
+`ISLANDORA_TAG` tells Docker what version of [Isle Buildkit](https://github.com/Islandora-Devops/isle-buildkit) 
+to use for the images. You should set this to the most 
+[recent release](https://github.com/Islandora-Devops/isle-buildkit/releases) number, 
+unless you have a reason to use older images.
+
+> [!WARNING]
+> You should not use `ISLANDORA_TAG=main` in production.
+
 If setting up your own images on a remote Docker image registry like [DockerHub],
 set the following line to your use your image registry: