Update build instructions.

README.md

@@ -188,6 +188,6 @@ Some cases may need secrets available in the environment, e.g. for S3 uploads to
 
 ## Building
 
-New images can be conveniently built on [Docker Hub](https://hub.docker.com/r/futurice/docker-volume-backup/~/settings/automated-builds/). Use the "Trigger" button with a tag name, for example:
+New images can be conveniently built on [Docker Hub](https://hub.docker.com/r/futurice/docker-volume-backup/~/settings/automated-builds/). Update the tag name, save, and use the "Trigger" button:
 
 ![Docker Hub build](doc/docker-hub-build.png)