Add Upgrade note for 1.8

Author: dbpolito

charts/generic/README.md

@@ -3,6 +3,16 @@
 Kool generic chart, you can deploy pretty much anything using this little guy.
 
 
+### Upgrading to 1.8
+
+In 1.8, we've fixed a bug in the Deployment labels and selector labels. As a result, the `matchLabels` of the Deployment have changed. Unfortunately, `matchLabels` are immutable (see https://handbook.giantswarm.io/docs/dev-and-releng/app-developer-processes/matchlabels-are-immutable/), so to complete the upgrade, you need to delete the Deployment before proceeding.
+
+To do this, run the following command:
+
+```bash
+kubectl delete deployment --namespace={namespace} --selector=app.kubernetes.io/instance={release_name} --selector='!app.kubernetes.io/deployment'
+```
+
 ### Upgrading to 1.4
 
 Service signature has changed in order to support multiple ports in the same service: