Add a pull request template for future changes. (#1712)

This should help reduce maintenance overhead when drafting new
releases. The test checkbox item is simply to have the developer
consider how to test their change. I'm less sure that will result
in significant change.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,12 @@
+# Description
+
+Please include a summary of the change and which issue is fixed. Please also
+include relevant motivation and context. Your commit message should include
+this information as well.
+
+Fixes # (issue)
+
+# Checklist:
+
+- [ ] I have added the relevant tests for this change.
+- [ ] I have added an item to the Pending section of ``docs/changes.rst``.