Add pre-commit description to contributing page

Author: BrianJKoopman

CONTRIBUTING.rst

@@ -53,3 +53,69 @@ Development Guide
 Contributors should follow the recommendations made in the `SO Developer Guide`_.
 
 .. _SO Developer Guide: https://simons1.princeton.edu/docs/so_dev_guide/
+
+pre-commit
+``````````
+As a way to enforce development guide recommendations we have configured
+`pre-commit`_.  While not required (yet), it is highly recommended you use this
+tool when contributing to socs. It will save both you and the reviewers time
+when submitting pull requests.
+
+You should set this up before making and commiting your changes. To do so make
+sure the ``pre-commit`` package is installed (it is in ``requirements.txt``)::
+
+    $ pip install -r requirements.txt
+
+Then run::
+
+    $ pre-commit install
+
+This will install the configured git hooks and any dependencies. Now, whenever
+you commit the hooks will run. If there are issues you will see them in the
+output. This may automatically make changes to your staged files.  These
+changes will be unstaged and need to be reviewed (typically with a ``git
+diff``), restaged, and recommitted. For example, if you have trailing
+whitespace on a line, pre-commit will prevent the commit and remove the
+whitespace. You will then stage the new changes with another ``git add <file>``
+and then re-run the commit. Here is the expected git output for this example:
+
+.. code-block::
+
+    $ vim demo.py
+    $ git status
+    On branch koopman/test-pre-commit
+    Changes not staged for commit:
+      (use "git add <file>..." to update what will be committed)
+      (use "git restore <file>..." to discard changes in working directory)
+        modified:   demo.py
+
+    no changes added to commit (use "git add" and/or "git commit -a")
+    $ git add demo.py
+    $ git commit
+    Check python ast.........................................................Passed
+    Fix End of Files.........................................................Passed
+    Trim Trailing Whitespace.................................................Failed
+    - hook id: trailing-whitespace
+    - exit code: 1
+    - files were modified by this hook
+
+    Fixing demo/demo.py
+
+    $ git status
+    On branch koopman/test-pre-commit
+    Changes to be committed:
+      (use "git restore --staged <file>..." to unstage)
+        modified:   demo.py
+
+    Changes not staged for commit:
+      (use "git add <file>..." to update what will be committed)
+      (use "git restore <file>..." to discard changes in working directory)
+        modified:   demo.py
+    $ git add -u
+    $ git commit
+
+**Note:** This is a new tool to this repo, and the flake8 output might still be
+somewhat strict. If there are warnings that you think should be ignored, please
+bring this up for discussion in a new issue.
+
+.. _pre-commit: https://pre-commit.com/