Add documentation on working with ScPCA portal data

[cbethell]

Issue Addressed Closes #313

What is the purpose of these changes?

Now that we have synced the workflows in this repo with what's on the ScPCA portal, it's a great time to incorporate documentation in this repo on using data downloaded from the portal with these workflows.

What changes did you make?

This PR adds additional-docs/working-with-scpca-portal-data.md to guide users on how to prepare their project metadata file when using data downloaded from the ScPCA portal. The main README and module README files are also updated here to link to the working-with-scpca-portal-data.md documentation.

Any comments, concerns, or questions important for reviewers

• Does this added documentation seem clear and easy to follow to you?
• Can you think of any additional details that may be beneficial if added to the documentation in this PR?

Checklist

Place an x in all boxes that you have completed.

• [ ] I have run the most recent version of the code
• [ ] If I am adding in reports or generating any plots, I have reviewed the necessary reports
• [x] I have added all necessary documentation (if applicable)

[cbethell]

Thanks for the review @allyhawkins! I've restructured things a bit as suggested so let me know what you think now -- especially on the section mentioning the required input_data_dir and it's structure, and the section on running the workflow for the additional modules.

[cbethell]

I've implemented those last couple of changes you suggested @allyhawkins! Let me know what you think of the added wording.