relocate git-secrets patterns and documentation

[rosskarchner]

After chat discussion, it was determined that we should move these patterns here, instead of transforming the purpose of the clouseau repository. I included a slightly re-worked version of the readme.

Additions

• A set of patterns for usage with git-secrets
• instructions for installation and usage.

Testing

1. See the installation and usage guidance in the included README.md.

Screenshots

Run against cato

Notes

• It currently goes a little crazy when run against cfgov-refresh. I wonder if the patterns are too broad?

Checklist

• [ ] Changes are limited to a single goal (no scope creep)
• [ ] Code can be automatically merged (no conflicts)
• [ ] Code follows the standards laid out in the CFPB development guidelines
• [ ] Passes all existing automated tests
• [ ] Any change in functionality is tested
• [ ] New functions are documented (with a description, list of inputs, and expected output)
• [ ] Placeholder code is flagged / future todos are captured in comments
• [ ] Visually tested in supported browsers and devices (see checklist below :point_down:)
• [ ] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• [ ] Reviewers requested with the Reviewers tool :arrow_right:

Testing checklist

Browsers

• [ ] Chrome
• [ ] Firefox
• [ ] Safari
• [ ] Internet Explorer 8, 9, 10, and 11
• [ ] Edge
• [ ] iOS Safari
• [ ] Chrome for Android

Accessibility

• [ ] Keyboard friendly
• [ ] Screen reader friendly

Other

• [ ] Is useable without CSS
• [ ] Is useable without JS
• [ ] Flexible from small to large screens
• [ ] No linting errors or warnings
• [ ] JavaScript tests are passing