Add pull request template.

https://help.github.com/articles/creating-a-pull-request-template-for-your-repository/

Author: benlk

.github/pull_request_template.md

@@ -0,0 +1,21 @@
+## Changes
+
+<!-- what changed? -->
+
+- 
+
+## Why
+
+<!-- why did it change? -->
+
+For #
+
+## Testing/Questions
+
+Questions that need to be answered before merging:
+
+- [ ] 
+
+Steps to test this PR:
+
+1.