README: update new major version SOP to include barriers

Create an update barrier for the last version of N-2 so all previous
versions have to go through that last version of N-2, which is
guaranteed to have the key for N.

Decision documented in coreos/fedora-coreos-tracker#480 (comment)

README.md

@@ -88,16 +88,21 @@ one easy way to do this is for now:
 - run `cosa fetch --update-lockfile`
 - commit only the new package entries
 
-## Moving to a new major version of Fedora
+## Moving to a new major version (N) of Fedora
 
-Updating this repo itself is easy:
+Updating this repo:
 
 1. bump `releasever` in `manifest.yaml`
 2. update the repos in `manifest.yaml` if needed
 3. run `cosa fetch --update-lockfile`
 4. PR the result
 
-Though there are also some releng-related knobs that may need changes:
+Update server changes:
+
+1. Set a new update barrier for N-2 on all streams.
+   See [discussion](https://github.com/coreos/fedora-coreos-tracker/issues/480#issuecomment-631724629).
+
+Release engineering changes:
 
 1. verify that the `f${releasever}-coreos-signing-pending` Koji tag has
    been created