Write a new pull request template (#2276)

The new template emphasizes what actions the author and reviewer should perform.

Author: ttuegel

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,28 @@
+Fixes #nnnn
 
 ---
 
-###### Reviewer checklist
+###### Review checklist
 
-- [ ] Test coverage: `stack test --coverage`
-- [ ] Public API documentation: `stack haddock`
+The author performs the actions on the checklist.
+The reviewer evaluates the work and checks the boxes as they are completed.
+
+-   [ ] **Summary**
+    Write a summary of the changes.
+    Explain what you did to fix the issue, and why you did it.
+    Present the changes in a logical order.
+    Instead of writing a summary in the pull request,
+    you may push a clean Git history.
+-   [ ] **Documentation**
+    Write documentation for new functions.
+    Update documentation for functions that changed,
+    or complete documentation where it is missing.
+-   [ ] **Tests**
+    Write unit tests for every change.
+    Write the unit tests that were missing before the changes.
+    Include any examples from the reported issue as integration tests.
+-   [ ] **Clean up**
+    The changes are already clean.
+    Clean up anything near the changes that you noticed while working.
+    This does not mean only spatially near the changes, but logically near:
+    any code that interacts with the changes!