confirm_upgrade: add note to read 'known issues'

upgrade.sh

@@ -275,6 +275,9 @@ confirm_upgrade() {
 	msg "This $VERB should work fine, but in the worst case your device"
 	msg "may not boot anymore. Make backups of important data first!"
 	msg
+	msg "Make sure to read the release blog post of $CHANNEL_NEW, it may"
+	msg "have a 'known issues' section: https://postmarketos.org/blog/"
+	msg
 	msg "(Script version: $SCRIPT_VERSION)"
 	msg