Explain how to close an issue within a PR message

- updated the headline
- updated the checklist numbers (was all "1.")
- converted the instructions into a markdown comment to make it
  clearer what is an instruction.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,15 @@
-## Pull Request Guidelines
+<!--
+Checklist for submitting a Pull Request
+
+Please remove this comment block before submitting.
 
 1. Please read our [contributor guidelines](https://github.com/kubernetes/kubernetes/blob/master/CONTRIBUTING.md).
-1. See our [developer guide](https://github.com/kubernetes/kubernetes/blob/master/docs/devel/development.md).
-1. Follow the instructions for [labeling and writing a release note for this PR](https://github.com/kubernetes/kubernetes/blob/master/docs/devel/pull-requests.md#release-notes) in the block below.
+2. See our [developer guide](https://github.com/kubernetes/kubernetes/blob/master/docs/devel/development.md).
+3. If you want this PR to automatically close an issue when it is merged,
+   add `fixes #<issue number>` or `fixes #<issue number>, fixes #<issue number>`
+   to close multiple issues (see: https://github.com/blog/1506-closing-issues-via-pull-requests).
+4. Follow the instructions for [labeling and writing a release note for this PR](https://github.com/kubernetes/kubernetes/blob/master/docs/devel/pull-requests.md#release-notes) in the block below.
+-->
 
 ```release-note
 * Use the release-note-* labels to set the release note state