Add data loading documentation to README

[hakbailey]

Why these changes are being introduced:

To provide documentation for developers on how and when various types of data get loaded into the system.

How this addresses that need:

• Adds a Data Loading section to the README with documentation on seed data and registrar data loads, and a placeholder for test data loads

Side effects of this change:

None

Relevant ticket(s):

• https://mitlibraries.atlassian.net/browse/ETD-217

Developer

• All new ENV is documented in README
• All new ENV has been added to Heroku Pipeline, Staging and Prod
• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)
• Stakeholder approval has been confirmed (or is not needed)

Code Reviewer

• The commit message is clear and follows our guidelines
  (not just this pull request message)
• There are appropriate tests covering any new functionality
• The documentation has been updated or is unnecessary
• The changes have been verified
• New dependencies are appropriate or there were no changes

Requires database migrations?

NO

Includes new or updated dependencies?

NO

[coveralls]

Coverage remained the same at 97.047% when pulling 0d47cc1 on etd-217-document-data-loading into a069d54 on main.

[JPrevost]

Please update syntax from rake to rails. Thanks for documenting this!