docs: upstream rebase guide

Document rebase steps and release steps

Signed-off-by: Hareesh Puthalath <hareeshp@nvidia.com>

Author: hareeshpc

docs/upstream-rebase-process.md

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+# Upstream Rebase Process
+
+This repo vendors a forked `ovn-kubernetes` repo as a git submodule at
+`ovn-kubernetes/`. The fork contains DPF-specific patches on top of upstream
+`ovn-kubernetes`. Use this document to rebase the fork on the latest upstream
+changes, update the submodule pointer in this repo, and push the results.
+
+## Prerequisites
+
+- You have cloned this repo locally.
+- The submodule has an `origin` remote (your fork); you will add the `upstream` remote during setup.
+- Your working trees (both the submodule and this repo) are clean.
+- You will rebase onto `main`, and use a forked branch for test/validation if needed.
+
+## Initialize and configure the submodule
+
+1. From the top-level repo, initialize and update the submodule:
+   - `git submodule update --init --recursive ovn-kubernetes`
+2. Enter the submodule:
+   - `cd ovn-kubernetes`
+3. Verify remotes:
+   - `git remote -v`
+4. If `upstream` is missing, add it:
+   - `git remote add upstream https://github.com/ovn-kubernetes/ovn-kubernetes`
+5. Fetch latest refs:
+   - `git fetch upstream --prune`
+   - `git fetch origin --prune`
+
+## Rebase the fork on upstream
+
+1. In the submodule, checkout `main` and rebase it on upstream:
+   - `cd ovn-kubernetes`
+   - `git checkout main`
+   - `git rebase upstream/master`
+2. Resolve conflicts as needed, then continue the rebase:
+   - `git status`
+   - `git add <resolved-files>`
+   - `git rebase --continue`
+3. If you need to run tests or validation on your fork, create/use a forked branch for that instead of `main`:
+   - `git checkout -b <fork-branch>`
+4. Push the rebased `main` branch back to your fork:
+   - `git push --force-with-lease origin main`
+
+Notes:
+
+- Use `--force-with-lease` because a rebase rewrites history.
+- If the fork branch is shared, coordinate the rebase with other users.
+
+## Update the submodule in this repo
+
+1. Return to the top-level repo:
+   - `cd ..`
+2. Update the submodule pointer to the new fork commit:
+   - `git status`
+   - `git add ovn-kubernetes`
+3. Commit and push the submodule update:
+   - `git commit -m "Bump ovn-kubernetes submodule"`
+   - `git push origin <branch>`
+
+## Tags
+
+The current taging strategy for the main branch is to use
+`v<year>.<month>.<date>-<sha of HEAD>`.
+
+for example `v26.1.27-ad5189a`
+
+## Release
+
+Follow the GitHub release process:
+
+1. Go to Releases -> Draft a new release.
+2. Pick a tag using the scheme in the Tags section and target `main`.
+3. Use the tag as the release title.
+4. Add a short summary of key changes and notable fixes, specifically a pointer to the ovn-kubernetes tag/sha.
+5. Mark it as a pre-release/latest release as needed.
+6. Publish the release.
+
+## Artifacts
+
+This results in the following artifcats.
+
+Helm chart repo: `oci://ghcr.io/mellanox/charts/ovn-kubernetes-chart`
+
+Helm chart version `v26.1.27-ad5189a`
+
+Image: `ghcr.io/mellanox/ovn-kubernetes-dpf-fedora:v26.1.27-ad5189a`