UPSTREAM: <carry>: Add document with procedure for rebasing

rebase.md

@@ -0,0 +1,72 @@
+# Rebasing Procedure
+
+This fork needs to be periodically synced with upstream changes. To do this we
+need to run the following steps:
+
+1. Ensure the *upstream*, *downstream* and your own fork of the repository are  
+   configured correctly as remotes.
+   ```bash
+   git remote -v
+   # if upstream or downstream are not present then add them
+   git remote add upstream https://github.com/kubernetes-sigs/aws-load-balancer-controller
+   git remote add downstream https://github.com/openshift/aws-load-balancer-controller
+   ```
+
+2. Ensure that the `main` branch is up-to-date with the *downstream* `main`
+   branch.
+   ```bash
+   git switch main
+   git pull downstream main
+   ```
+
+3. Update the remotes
+   ```bash
+   git remote update
+   ```
+
+4. Checkout the commit from upstream to which you want to rebase the `main`
+   branch.
+   ```bash
+   git checkout $TAG_OR_COMMIT_ID
+   git switch -c rebase
+   ```
+
+5. Merge the `main` branch into the current branch with
+   the [ours](https://git-scm.com/docs/merge-strategies#_merge_strategies)
+   merge strategy.
+   ```bash
+   git merge -s ours main
+   ```
+   _Note:_ This strategy should not be confused with the `ours` option for 
+   the `ort` merge strategy.
+
+7. Determine the changes present in the `main` branch which need to be
+   cherry-picked.
+   ```bash
+   git log --oneline --no-merges $TAG_OR_COMMIT_ID..main
+   ```
+
+## Message format for cherry-picked commits
+
+The commits which are cherry-picked into the rebase branch should have the 
+following format:
+
+* `UPSTREAM: <carry>: $MESSAGE`
+  A persistent carry that should probably be picked for the subsequent rebase
+  branch. In general, these commits are used to modify behavior from upstream.
+
+* `UPSTREAM: <drop>: $MESSAGE`
+  A carry that should probably not be picked for the subsequent rebase branch.
+  In general, these commits are used to maintain the codebase in ways that are
+  branch-specific, like the update of generated files or dependencies.
+
+* `UPSTREAM: 77870: $MESSAGE`
+  The number identifies a PR in the upstream repository(
+  i.e. https://github.com/kubernetes-sigs/aws-load-balancer-controller/pull/<pr_id>)
+  A commit with this message should only be picked into the subsequent rebase
+  branch if the commits of the referenced PR are not included in the upstream
+  branch. To check if a given commit is included in the upstream branch, open
+  the referenced upstream PR and check any of its commits for the release tag (
+  e.g. v.1.20.0) targeted by the new rebase branch.
+
+