Update README with branch info

README.md

@@ -5,7 +5,14 @@ a A lightweight extensible base docker image, suitable for development or produc
 This image offers relatively no useful functionality on its own, but serves well as the base of other images. For an example how we extend this image, please see [unb-libraries/docker-nginx](https://github.com/unb-libraries/docker-nginx).
 
 ## Branches/Tags
-The only currently maintained tag is **ghcr.io/unb-libraries/base:2.x**.
+The only currently maintained branches are:
+
+|                  Tag                      | Alpine |
+|:-----------------------------------------:|--------|
+|  ghcr.io/unb-libraries/base:3.18.x        | 3.18   |
+|  ghcr.io/unb-libraries/base:3.17.x        | 3.17   |
+
+Other branches may exist for legacy purposes, but should not be used.
 
 ## Author / Contributors
 This application was created at [![UNB Libraries](https://github.com/unb-libraries/assets/raw/master/unblibbadge.png "UNB Libraries")](https://lib.unb.ca) by the following humans: