Link docs to upgrading section of chart readme

Signed-off-by: Scott Rigby <[email protected]>

Author: scottrigby

docs/deploy/upgrade.md

@@ -47,3 +47,7 @@ you should be able to upgrade using
 ```shell
 helm upgrade --reuse-values ngx-ingress ingress-nginx/ingress-nginx
 ```
+
+### Migrating from stable/nginx-ingress
+
+See detailed steps in the upgrading section of the `ingress-nginx` chart [README](https://github.com/kubernetes/ingress-nginx/blob/master/charts/ingress-nginx/README.md#migrating-from-stablenginx-ingress).