Updating Prow readme

Author: aireilly

scripts/prow/README.adoc

@@ -10,6 +10,23 @@ To add a new job to Prow CI, do the following:
 . Install link:https://podman.io/docs/installation[Podman].
 
 . Fork and clone the https://github.com/openshift/release repository to `$HOME/release`.
++
+[NOTE]
+====
+If you have already forked the repository, make sure it is it is up to date.
+Run the following commands:
+
+[source,terminal]
+----
+cd $HOME/release
+git remote add upstream git@github.com:openshift/release.git
+git checkout master
+git fetch upstream
+git rebase upstream/master
+git checkout <your_new_branch> <1>
+----
+<1> Replace `<your_new_branch>` with an appropriate branch name.
+====
 
 . Create the new branch config by running the `add-new-prow-config.sh` script passing the `$VERSION`, `$BRANCH` and `$DISTROS` variables.
 Open a shell prompt in the `openshift-docs/` directory, and run the script passing in the required variables, for example: