Add a GitHub pull request template

.github/PULL_REQUEST_TEMPLATE

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+## Summary
+
+What does this pull request do?
+
+**To do:**
+
+If this pull request is a work-in-progress, add a checklist here of
+the remaining tasks so it's clear what still needs to be done.
+
+**Related issues:**
+
+If this pull request is related to any other issues, list them here.
+
+
+## Checklist
+
+- [ ] Travis builds the PR successfully
+
+**If this is fixing a bug:**
+
+- [ ] You have added a regression test to dejafu-tests
+
+**If this is adding a feature:**
+
+- [ ] You have added new tests to dejafu-tests
+
+
+## Benchmark results (for performance issues)
+
+**Before:**
+
+```
+$ stack exec -- dejafu-tests +RTS -s
+
+...put your results here...
+```
+
+**After:**
+
+```
+$ stack exec -- dejafu-tests +RTS -s
+
+...put your results here...
+```