docs: create runbook to backport changes to older versions (#330)

This PR contains runbook with backporting changes to older versions.

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Co-authored-by: Artem Rys <[email protected]>
Co-authored-by: Marcin Bruzda <[email protected]>

runbooks/backporting-changes-to-older-version.md

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+# Runbook to backport changes to previous versions of `addonfactory-workflow-addon-release` 
+`addonfactory-workflow-addon-release` is utilized by all supported TAs. While it is strongly recommended to use the latest minor version of the reusable action, not all TAs consistently follow this guideline. As a result, there are cases when crucial updates introduced in the latest reusable workflow version need to be backported to its older versions that are still in use.
+
+This runbook shows a real example of backporting changes correlated to `ta-automation-k8s-manifests`. In the example: 
+- current version of `addonfactory-workflow-addon-release` is `v4.17.0`
+- there was a bug in `ta-automation-k8s-manifests` affecting all TAs
+  - there is a need to make a fix and release `v4.17.1` containing fixed `ta-automation-k8s-manifests` version ([PR](https://github.com/splunk/addonfactory-workflow-addon-release/pull/329))
+  - there is need to backport it to `v4.16` (the old version which is still in use by some TAs)
+  - the latest patch release of `v4.16` is `v4.16.14`
+### Steps 
+- make release `v4.17.1` with necessary changes
+- fetch all existing tags from `addonfactory-workflow-addon-release`: 
+  ```
+  git checkout main
+  git pull
+  git fetch --tags
+  ```
+- checkout to the latest tag of the minor release you want to backport the changes to
+  ```
+  git checkout v4.16.14
+  ```
+- create a new branch based on the tag you are currently checked out to
+  ```
+  git checkout -b fix/bump-k8s-manifest-version
+  ```
+- changes made in `ta-automation-k8s-manifests` were correlated with changes in `addonfactory-workflow-addon-release`, so there is a need to backport **only necessary** changes to `v4.16`. There are two ways to do that: by cherrypicking specific commits (and resolving the conflicts if they exist) or by commiting necessary changes manually.
+  - for example make necessary changes and commit them: 
+    ```
+    git add .github/workflows/reusable-build-test-release.yml
+    git commit -m "fix: bump k8s-manifest version"
+    ```
+- push newly created branch to the remote repository
+  - ```git push -u origin fix/bump-k8s-manifest-version```
+- in GitHub UI: 
+  - navigate to releases
+  - draft a new release 
+  - `Choose a tag`: type the tag that will be created, i.e. `v4.16.15` 
+  - `Target` - newly created branch `fix/bump-k8s-manifest-version`
+  - click on `Generate release notes` 
+  - `Title of release` should be `v4.16.15 backport` 
+  - write description of the changes 
+  - uncheck `Set as the latest release` box 
+  - click `Publish release`
+  - check if the release is available, and it points at the proper version - https://github.com/splunk/addonfactory-workflow-addon-release/tags tag `v4.16` should point to the same commit as tag `v4.16.15` 
+    <img src="images/backporting/compare-tags.png" alt="tags"/>
+- run the workflow for some TA using v4.16 to verify if the pipeline works as expected.