Add kind label to pull request template

Currently, the template doesn't prompt contributors to add the kind
label. So many times the check fails and user has to add it after
getting the test for kind label fails.

This change ensures that the PR template prompts contributors to do all
the required tasks.

Fixes #3397

.github/pull_request_template.md

@@ -2,8 +2,15 @@
 
 # Changes
 
-<!-- Describe your changes here- ideally you can get that description straight from
-your descriptive commit message(s)! -->
+<!-- 
+Describe your changes here- ideally you can get that description straight from
+your descriptive commit message(s)! 
+
+In addition, categorize the changes you're making using the "/kind" Prow command, such as:
+/kind bug
+/kind documentation
+/kind feature
+-->
 
 # Submitter Checklist