Add issue link info to PR template

While reviewing pull requests I noticed there are a lot of PRs that
are addressing open issues, but the PRs are not properly linked. This
results in a lot of issues being left open, even after the fix has
been merged.

This updates the PR template to give a short instruction on adding the
expected line in the PR description so that the issue and PR can be
linked automatically and closed on merge.

Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,11 +1,6 @@
 <!--
-
  Hello!
 
- Remember to ADD A DESCRIPTION and delete this note before submitting
- your pull request. The description should explain what will change,
- and why.
-
  PLEASE title the FIRST commit appropriately, so that if you squash all
  your commits into one, the combined commit message makes sense.
  For overall help on editing and submitting pull requests, visit:
@@ -20,5 +15,24 @@
 
  If you're documenting a feature that will be part of a future release, see
  https://kubernetes.io/docs/contribute/new-content/new-features/ for advice.
-
 -->
+### Description
+
+<!--
+ Remember to ADD A DESCRIPTION and delete this note before submitting
+ your pull request. The description should explain what will change,
+ and why.
+-->
+
+### Issue
+
+<!--
+ If this pull request resolves an open issue, please link the issue in the PR
+ description so it will automatically close when the PR is merged.
+
+ See the GitHub documentation for more details and other options:
+
+ https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword
+-->
+
+Closes: #