Document the extensions that we provide

[kylewillmon]

We need to add documentation for the extensions that we provide. These can be documented directly in the repository, with a link in our extensions docs on readme.com

Acceptance Criteria

• Add a README.md file to the top-level extensions folder that contains a list of our extensions with a brief description of each
• Add a README.md to each extension folder that explains the extension in greater detail including examples of how to run the extension and any supported arguments, etc.
• Add a section to docs/extensions/extension_overview.md that will tell users that we have written a small set of extensions and include a link to the folder on GitHub for more information.