Add a pull request template (#1617)

* Add a pull request template

* Add a testing section

Author: itingliu

.github/pull_request_template.md

@@ -0,0 +1,18 @@
+_[One line description of your change]_
+
+### Motivation:
+
+_[Explain here the context, and why you're making that change. What is the problem you're trying to solve. References to issues the changes resolve, if any.]_
+
+### Modifications:
+
+_[Describe the modifications you've done.]_
+
+### Result:
+
+_[After your change, what will change.]_
+
+### Testing:
+
+_[The specific testing that has been done or needs to be done to further
+  validate any impact of the changes.]_