Create PULL_REQUEST_TEMPLATE.md (#2328)

- Add a template for pull requests.

Author: Svetlana Karslioglu

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,11 @@
+Fixes #ISSUE_NUMBER
+
+## Description
+<!--- Describe your changes in detail -->
+
+## Checklist
+<!--- Make sure to add `x` to all items in the following checklist: -->
+- [ ] The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
+- [ ] Only one issue is addressed in this pull request
+- [ ] Labels from the issue that this PR is fixing are added to this pull request
+- [ ] No unnessessary issues are included into this pull request.