Add a GitHub PR template

Author: hristog

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,8 @@
+- [ ] Closes #xxxx
+- [ ] Tests added / passed
+- [ ] Passes `flake8`
+- [ ] Passes `black dask_ml`
+- [ ] Passes `isort .`
+- [ ] Passes `mypy dask_ml/metrics`
+- [ ] Passes `mypy dask_ml/preprocessing`
+