Add section about 'contrib' module

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If non-trivial please provide a summary of the changes here.

Submitters and reviewers please make sure

• internal links are relative and to the .md file
• avoid links named "here"
• alt-text for images is descriptive
• doesn't duplicate existing content
• spell check
• renders correctly
• does not include any identifiable information (real usernames, project names, etc)

-- I do include some project information, but I wonder if it's appropriate in this case.

This MR adds a section about the contrib module.