feat: add instructions for adding our policy.json (#300)

* feat: add instructions for adding our policy.json

* fix: recommend switching to an unsigned image first

* fix: clarify wording

* fix: incorrect bullet

* fix: update to confirmed working command

docs/images.md

@@ -7,6 +7,15 @@ This list is in alphabetical order.
 
     If you're coming from an existing Fedora installation **please ensure you remove layered packages before rebasing!** If you're not familiar with the usage of rpm-ostree we strongly recommend [reading the documentation](https://coreos.github.io/rpm-ostree/).
 
+## Preparing to Rebase
+
+1. `rpm-ostree reset` will remove all your layered packages and prepare for rebasing. 
+2. Rebase to an *unsigned* Universal Blue image, this will ensure that you have the proper keys and policies on your machine:
+
+        rpm-ostree ostree-unverified-registry:ghcr.io/ublue-os/silverblue-main
+
+3. Then reboot, next rebase to a signed image of your choice below: 
+
 ## Image List
 
 {!_generated_image_list.md!}