Add pre-commit config and apply changes (#54)

- Add .pre-commit-config.yaml with initial config.
- Add python black pre-commit config
- Apply pre-commit code changes
- Add information to CONTRIBUTING.md
- Add pre-commit to build requirements
- Run pre-commit during travis CI build
- Use a travis build matrix to control when pre-commit runs
  in the CI process. Use explicit include to only run
  pre-commit on Python 3.9.

CONTRIBUTING.md

@@ -8,7 +8,7 @@ I love your input! I want to make contributing to this project as easy and trans
 - Becoming a maintainer
 
 ## I use [Github Flow](https://docs.github.com/en/get-started/quickstart/github-flow), so all code changes happen through pull requests
-Pull requests are the best way to propose changes to the codebase (I use 
+Pull requests are the best way to propose changes to the codebase (I use
 [Github Flow](https://docs.github.com/en/get-started/quickstart/github-flow)). I actively welcome your pull requests:
 
 1. Fork the repo and create your branch from `master`
@@ -20,7 +20,7 @@ Pull requests are the best way to propose changes to the codebase (I use
 
 ## Any contributions you make will be under the MIT Software License
 When you submit code changes, your submissions are understood to be under the same
-[MIT License](http://choosealicense.com/licenses/mit/) that covers the project. Feel free to contact the maintainers 
+[MIT License](http://choosealicense.com/licenses/mit/) that covers the project. Feel free to contact the maintainers
 if that's a concern.
 
 ## Report bugs using Github's [issues](https://github.com/jongracecox/anybadge/issues)
@@ -48,6 +48,42 @@ By contributing, you agree that your contributions will be licensed under its MI
 
 # Technical stuff
 
+## Pre-commit
+This projects makes use of [pre-commit](https://pre-commit.com) to add some safety checks and create consistency
+in the project code. When committing changes to this project, please first [install pre-commit](https://pre-commit.com/#install),
+then activate it for this project:
+
+```bash
+pip install pre-commit
+pre-commit install
+```
+
+After installing pre-commit to your project (with `pre-commit install`), committing to the project will trigger a series
+of checks, and fixers. This process may reject your commit or make changes to your code to bring it into line with the
+project standards. For example, [Python black](https://github.com/psf/black) will be used to reformat any code. When
+changes are made by these pre-commit hooks you will need to re-add and commit those changes in order for pre-commit to
+pass.
+
+Here is some example output from pre-commit:
+
+```
+trim trailing whitespace.................................................Failed
+- hook id: trailing-whitespace
+- exit code: 1
+- files were modified by this hook
+
+Fixing tests/test_anybadge.py
+
+fix end of files.........................................................Failed
+- hook id: end-of-file-fixer
+- exit code: 1
+- files were modified by this hook
+
+Fixing examples/color_teal.svg
+```
+
+This shows that two files were updated by hooks, and need to be re-added (with `git add`) before trying to commit again.
+
 ## Documentation
 The `README.md` file contains a table showing example badges for the different built-in colors. If you modify the
 appearance of badges, or the available colors please update the table using the following code: