Update PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

@@ -7,14 +7,24 @@ Include a summary of major changes in bullet points:
 
 ## Additional dependencies introduced (if any)
 
-* List all new dependencies needed and justify why. While adding dependencies that bring
-  significantly useful functionality is perfectly fine, adding ones that add trivial
-  functionality, e.g., to use one single easily implementable function, is frowned upon.
-  Provide a justification why that dependency is needed. Especially frowned upon are
-  circular dependencies.
+* List all new dependencies needed and justify why.
 
 ## TODO (if any)
 
 If this is a work-in-progress, write something about what else needs to be done.
 
 * Feature 1 supports A, but not B.
+
+## Checklist
+
+Before a pull request can be merged, the following items must be checked:
+
+* [ ] Make sure your code is properly formatted. [isort](https://pycqa.github.io/isort/) and [black](https://black.readthedocs.io/en/stable/getting_started.html) are used for this purpose. The simplest way is to use [pre-commit](https://pre-commit.com). See instructions [here](https://github.com/openkim/kliff/blob/main/docs/source/contributing_guide.md#code-style).
+* [ ] Doc strings have been added in the [Google docstring format](https://www.sphinx-doc.org/en/master/usage/extensions/example_google.html) on your code.
+* [ ] Type annotations are **highly** encouraged. Run [mypy](http://mypy-lang.org) to
+  type check your code.
+* [ ] Tests have been added for any new functionality or bug fixes.
+* [ ] All linting and tests pass.
+
+Note that the CI system will run all the above checks. But it will be much more
+efficient if you already fix most errors prior to submitting the PR.