Add item to update the kubernetes updates doc (#257)

* Add item to update the kubernetes updates doc

Customers refer to this page before upgrading Sourcegraph. https://docs.sourcegraph.com/admin/updates/kubernetes
Any manual migration steps to this Helm repo should be documented on that page in addition to the Changelog in this repo.

* remove space

Author: gabtorre

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,6 +2,7 @@
 
 - [ ] Follow the [manual testing process](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/TEST.md)
 - [ ] Update [changelog](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/charts/sourcegraph/CHANGELOG.md)
+- [ ] Update [Kubernetes update doc](https://docs.sourcegraph.com/admin/updates/kubernetes)
 
 ### Test plan