Add image autobumper documentation (#12205)

* Add image autobumper documentation

* Apply suggestions from code review

Co-authored-by: Iwona Langer <[email protected]>

* Fix documentation after review

---------

Co-authored-by: Iwona Langer <[email protected]>

cmd/image-autobumper/README.md

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+# Image Autobumper
+
+Image Autobumper is a tool for automatically updating the version of a Docker image in a GitHub repository.
+
+Key features:
+* Automatically detects the latest Docker image version  in a GitHub repository
+* Automatically detects places in the file where image URLs are defined
+* Automatically updates the image version in the repository based on the provided configuration
+* Automatically finds `.yaml` and `.yml` files in the repository
+* Supports any file format provided in `extraFiles` option.
+
+## Quickstart Guide
+
+To use Image Autobumper in your repository, create a GitHub workflow that references the Image Autobumper reusable workflow.
+
+See the following examples:
+* [GitHub workflow updating the Docker image version using Image Autobumper](https://github.com/kyma-project/test-infra/blob/main/.github/workflows/autobump-images.yml)
+* [Configuration file for Image Autobumper](https://github.com/kyma-project/test-infra/blob/main/configs/autobump-config/test-infra-autobump-config.yaml)
+
+## Supported Events
+
+Image Autobumper supports all events that trigger a GitHub workflow except the **pull_request** event. That limitation is caused by the requirement to fetch the `kyma-bot` token, which is used to access the forked repository. The **pull_request** event does not provide the OIDC token required to authenticate against Google Cloud Secret Manager.
+
+## Reusable Workflow Reference
+
+The workflow that uses Image Autobumper reusable workflow must use the exact reference to the reusable workflow. The value of the **uses** key must be `kyma-project/test-infra/.github/workflows/image-autobumper.yml@main`.
+
+```yaml
+uses: kyma-project/test-infra/.github/workflows/image-builder.yml@main
+```
+
+> [!WARNING]
+> Using different references to the reusable workflow results in an error during the workflow execution.
+
+## Reusable Workflow Inputs
+
+The Image Autobumper reusable workflow accepts inputs to parametrize the build process. See the accepted inputs description in the [`image-autobumper` reusable workflow](/.github/workflows/image-autobumper.yml) file.