Add readme for codecov proxy action

proxy-actions/codecov/README.md

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+# Codecov GitHub Action Proxy
+
+This is a proxy action that wraps the official [Codecov GitHub Action](https://github.com/codecov/codecov-action).
+
+## Updating to a New Version
+
+When a new version of the Codecov action is released, follow these steps:
+
+### 1. Copy Inputs from original action
+- Visit the [Codecov action repository](https://github.com/codecov/codecov-action/blob/main/action.yml)
+- Copy all input definitions from the upstream `action.yml`
+- Paste them into the `inputs` section of this action's `action.yml`
+
+### 2. Update the Composite Action Step
+- For any new inputs, add corresponding `with:` entries in the composite step that passes them through to the codecov action
+- For any removed inputs, remove the corresponding `with:` entries
+- Ensure the `uses:` reference is updated to the new version git hash and tag in the comment
+
+### 3. Update the Version Pin
+- Update the `version:` input in the composite step to the new release version
+- **⚠️ Note:** `version: v11.2.3` is currently pinned and should not be bumped until this issue is resolved: https://github.com/getsentry/prevent-cli/issues/101
+
+## Example
+
+The `action.yml` file defines all available inputs that can be passed to this proxy action, which are then forwarded to the upstream Codecov action in the composite step.