Merge pull request #531 from voxel51/jleven-patch-1

Modify PR template to provide more context on priority

Author: jleven

.github/pull_request_template.md

@@ -2,7 +2,10 @@
 
 <!-- Explain why you are making this change. Describe the problem. -->
 
-<!-- Please also assign a priority label to the PR. -->
+<!-- Please check a priority box below, and also assign a priority label
+to the PR. Definitions for each priority are on its label.
+-->
+
 Review Priority
 
 * [ ] high