Modify PR template to provide more context on priority

Signed-off-by: Joshua Leven <josh@voxel51.com>

Author: jleven

.github/pull_request_template.md

@@ -2,7 +2,7 @@
 
 <!-- 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 each label. -->
 Review Priority
 
 * [ ] high