docs: add instructions for using Renovate

Our versioning schema doesn't work with Renovate out of the box.
Fortunately it can be configured to handle it fine. Here we add
instructions for users so that they can set their Renovate configuration
up correctly.

Author: iainlane

README.md

@@ -9,6 +9,32 @@ themselves.
 [scorecard]: https://scorecard.dev/viewer/?uri=github.com/grafana/shared-workflows
 [scorecard image]: https://api.scorecard.dev/projects/github.com/grafana/shared-workflows/badge
 
+## Custom Renovate config
+
+This is a monorepo containing several Actions. When we release a workflow, we create a tag `<workflow name>/v<workflow version>`.
+
+While Dependabot can update references to these actions, Renovate can't do it out of the box. It can, however, be configured to do so:
+
+```json5
+{
+  packageRules: [
+    {
+      matchPackageNames: ["grafana/shared-workflows"],
+      versioning: "regex:^(?<compatibility>.*)[-/]v?(?<major>\\d+)\\.(?<minor>\\d+)\\.(?<patch>\\d+)?$",
+
+      // By default, the dependency name is the same as the package name. For
+      // Actions, this is `org/repo`. This means that we'd get all actions from
+      // `shared-workflows` in the same branch. Our tag scheme contains the name
+      // of the action being update to as well, so we can grab it from there and
+      // set the dep name, then put this in the branch name. This should mean
+      // that each shared workflow is handled separately.
+      overrideDepName: 'grafana/shared-workflows/{{ lookup (split newVersion "/") 0 }}',
+      additionalBranchPrefix: "{{depName}}",
+    },
+  ],
+}
+```
+
 ## Notes
 
 ### Configure your IDE to run Prettier
@@ -40,7 +66,7 @@ include a tag in a commend after the SHA, it can update the comment too. For
 example:
 
 ```yaml
-- uses: action/foo@abcdef0123456789abcdef0123456789 # v1.2.3
+- uses: action/foo@abcdef0123456789abcdef0123456789 # foo-action/v1.2.3
 ```
 
 [hardening]: https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-third-party-actions