Mention CONTRIBUTING.md and unit tests in the GitHub PR template

Author: jamesderlin

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,10 +1,23 @@
+<!--
+INSTRUCTIONS:
+
+Please read the CONTRIBUTING.md file first.  In particular, changes to code
+behavior should include unit tests.
+-->
+
 ## Description
 
-*Replace this paragraph with a description of what this PR is doing. If you're modifying existing behavior, describe the existing behavior, how this PR is changing it, and what motivated the change. If you're changing visual properties, consider including before/after screenshots (and runnable code snippets to reproduce them).*
+<!--
+Replace this with a description of what this PR is doing. If you're modifying existing behavior, describe the existing behavior, how this PR is changing it, and what motivated the change. If you're changing visual properties, consider including before/after screenshots (and runnable code snippets to reproduce them).
+-->
+*TODO*
 
 ## Related Issues
 
-*Replace this paragraph with a list of issues related to this PR. Indicate which of these issues are resolved or fixed by this PR.*
+<!--
+Replace this with a list of issues related to this PR. Indicate which of these issues are resolved or fixed by this PR.
+-->
+*TODO*
 
 ## Checklist