chore: update PR template for breaking changes (#28698)

The team had feedback during a sprint retro that the breaking change
process documentation was a bit spread out. In particular, some team
members were not clear that you need to a) update the breaking.md file
and b) add the "BREAKING CHANGE" comment to the commit description.

This PR adds both of these things are comments for awareness.

.github/PULL_REQUEST_TEMPLATE.md

@@ -21,7 +21,12 @@ Issue number: resolves #
 - [ ] Yes
 - [ ] No
 
-<!-- If this introduces a breaking change, please describe the impact and migration path for existing applications below. -->
+<!--
+  If this introduces a breaking change:
+  1. Describe the impact and migration path for existing applications below.
+  2. Update the BREAKING.md file with the breaking change.
+  3. Add "BREAKING CHANGE: [...]" to the commit description when merging. See https://github.com/ionic-team/ionic-framework/blob/main/.github/CONTRIBUTING.md#footer for more information.
+-->
 
 
 ## Other information