Add homepage for change-ns plugin (#95)

/ref #92 /cc @juanvallejo 

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**Checklist for plugin developers:**

- [x] Read the [Plugin Naming Guide](https://github.com/GoogleContainerTools/krew/tree/master/docs/NAMING_GUIDE.md) (for new plugins)
- [x] Verify the installation from URL or a local archive works (`kubectl krew install --manifest=[...] --archive=[...]`)

plugins/change-ns.yaml

@@ -35,6 +35,7 @@ spec:
         os: windows
         arch: amd64
   version: v1.0.0
+  homepage: https://github.com/juanvallejo/kubectl-ns
   caveats: This plugin requires an existing KUBECONFIG file, with a `current-context` field set.
   shortDescription: View or change the current namespace via kubectl.
   description: View or change the current namespace in your KUBECONFIG file. Honors global config flags found in kubectl, such as `--user`, `--cluster`, `--context`, and `--namespace`.