Add CONTRIBUTING.md

CONTRIBUTING.md

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+## Update image gathering scripts
+
+For every release, we'll need to update the image gather scripts for fetching the images used by Knative Charms.
+
+To get an overview of how the script works please read https://github.com/canonical/knative-operators/issues/142
+The script has a limitation on how it's deducing images from upstream manifests, which is tracked in
+https://github.com/canonical/knative-operators/issues/220
+
+The script also needs to gather images for `net-istio`, that has it's own release cadance (although almost 1-1 with Knative Serving, but sometimes tags for Serving might not exist for `net-istio`).
+
+**Process:**
+1. Confirm the Knative Serving version you want to update `net-istio` for
+2. Deduce the `net-istio` that should be used
+  * If you update based on a Kubeflow release, then you can deduce this from [upstream Kubeflow manifests](https://github.com/kubeflow/manifests/blob/v1.9-branch/common/knative/README.md?plain=1#L8) (make sure to checkout to correct release branch)
+  * If you update to an arbitrary Knative Serving version, then check the branches of [`net-istio`](https://github.com/knative-extensions/net-istio/tags) and pick the closest one to your Knative Serving release
+3. Update the `tools/get-images.sh` script to use the selected version of `net-istio`
+  * If the `net-istio` tag is not 1-1 with the Serving version then hardcode the version of `net-istio` in the script