Add pull request template

Author: FranzBusch

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,13 @@
+_[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.]_
+
+### Modifications:
+
+_[Describe the modifications you've done.]_
+
+### Result:
+
+_[After your change, what will change.]_