Adding DOI to CITATION.cff and README.md

[fletchapin]

Pull request recommendations:

• Name your pull request {your development type}: {short description}. Ex: Feature: flocculator validation
• Link to any relevant issue in the PR description. Ex: Resolves Remove all hard-coded constants from utility files #12, adds flocculator validation
• Provide context of changes.
• Provide relevant tests for your feature or bug fix.
• Provide or update documentation for any feature added by your pull request.

Thanks for contributing!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!