Add pre-commit instructions to README [#39]

Syncs documentation with other repos that use `pre-commit`.

README.md

@@ -138,3 +138,24 @@ For more locally testable scripts, Cram-style functional tests live in `tests` a
 
 1. Download Cram: `pip install cram`
 2. Run the tests: `cram tests/`
+
+## Working on this repo
+
+This repo is configured to use [pre-commit](https://pre-commit.com),
+to help automatically catch common coding errors and syntax issues
+with changes before they are committed to the repo.
+.
+If you will be writing new code or otherwise working within this repo,
+please do the following to get started:
+
+1. install `pre-commit` by running either `python -m pip install
+   pre-commit` or `brew install pre-commit`, depending on your
+   preferred package management solution
+2. install the local git hooks by running `pre-commit install` from
+   the root of the repo
+3. when problems are detected, correct them in your local working tree
+   before committing them.
+
+Note that these pre-commit checks are also run in a GitHub Action when
+changes are pushed to GitHub, so correcting issues locally will
+prevent extra cycles of correction.