Update PR template to encourage linking to issues (#9023)

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,25 @@
 ### Required for all PRs:
 
-- [ ] Associated README.md updated.
+<!-- Complete the tasks in the following list. Change [ ] to [x] to
-- [ ] Has appropriate unit tests.
+show completion. -->
+
+- [ ] Updated associated README.md.
+- [ ] Wrote appropriate unit tests.
+
+<!-- Link to issues that describe the need for the change. Issues
+should include context that will help reviewers understand why the
+change is needed.
+
+Make sure to link issues and using a keyword like "resolves #1234".
+https://docs.github.com/en/github/managing-your-work-on-github/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword
+-->
+
+resolves #
+
+<!-- Finally, include a summary of the code change itself. This
+description should tell reviewers how the issues were resolved.
+
+example: Fixed an off by one error in counter variable in type FooBar.
+
+example: Added an input plugin to gather yak shaving metrics using
+golang library yaktech/shaver. -->