Add PR checklist to contributing docs (#3226)

* Draft of PR checklist.

* Clean up and formatting.

* Added pre-commit suggestion.

Co-Authored-By: Maximilian Roos <[email protected]>

* Added PR checklist.

Author: Gregory Gundersen

doc/contributing.rst

@@ -842,3 +842,22 @@ branch has not actually been merged.
 The branch will still exist on GitHub, so to delete it there do::
 
     git push origin --delete shiny-new-feature
+
+
+PR checklist
+------------
+
+- **Properly comment and document your code.** See `"Documenting your code" <https://xarray.pydata.org/en/stable/contributing.html#documenting-your-code>`_.
+- **Test that the documentation builds correctly** by typing ``make html`` in the ``doc`` directory. This is not strictly necessary, but this may be easier than waiting for CI to catch a mistake. See `"Contributing to the documentation" <https://xarray.pydata.org/en/stable/contributing.html#contributing-to-the-documentation>`_.
+- **Test your code**.
+
+    - Write new tests if needed. See `"Test-driven development/code writing" <https://xarray.pydata.org/en/stable/contributing.html#test-driven-development-code-writing>`_.
+    - Test the code using `Pytest <http://doc.pytest.org/en/latest/>`_. Running all tests (type ``pytest`` in the root directory) takes a while, so feel free to only run the tests you think are needed based on your PR (example: ``pytest xarray/tests/test_dataarray.py``). CI will catch any failing tests.
+
+- **Properly format your code** and verify that it passes the formatting guidelines set by `Black <https://black.readthedocs.io/en/stable/>`_ and `Flake8 <http://flake8.pycqa.org/en/latest/>`_. See `"Code formatting" <https://xarray.pydata.org/en/stablcontributing.html#code-formatting>`_. You can use `pre-commit <https://pre-commit.com/>`_ to run these automatically on each commit.
+
+    - Run ``black .`` in the root directory. This may modify some files. Confirm and commit any formatting changes.
+    - Run ``flake8`` in the root directory. If this fails, it will log an error message.
+
+- **Push your code and** `create a PR on GitHub <https://help.github.com/en/articles/creating-a-pull-request>`_.
+- **Use a helpful title for your pull request** by summarizing the main contributions rather than using the latest commit message. If this addresses an `issue <https://github.com/pydata/xarray/issues>`_, please `reference it <https://help.github.com/en/articles/autolinked-references-and-urls>`_.

doc/whats-new.rst

@@ -91,6 +91,13 @@ Bug fixes
 
 .. _whats-new.0.12.3:
 
+Documentation
+~~~~~~~~~~~~~
+
+- Created a `PR checklist <https://xarray.pydata.org/en/stable/contributing.html/contributing.html#pr-checklist>`_ as a quick reference for tasks before creating a new PR
+  or pushing new commits.
+  By `Gregory Gundersen <https://github.com/gwgundersen>`_.
+
 v0.12.3 (10 July 2019)
 ----------------------