Update contribution.md

[aaraney]

Closes #143.

Updates to the contribution guide to reflect changes we've made to the package structure.

Changes

• Updates to the contribution guide to reflect changes we've made to the package structure.

Checklist

• [x] PR has an informative and human-readable title
• [x] PR is well outlined and documented. See #12 for an example
• [x] Changes are limited to a single goal (no scope creep)
• [x] Code can be automatically merged (no conflicts)
• [x] Code follows project standards (see CONTRIBUTING.md)
• [x] Passes all existing automated tests
• [x] Any change in functionality is tested
• [x] New functions are documented (with a description, list of inputs, and expected output) using numpy docstring formatting
• [x] Placeholder code is flagged / future todos are captured in comments
• [x] Reviewers requested with the Reviewers tool :arrow_right:

[jarq6c]

Thanks for this!