Add note to Simple Forms API README about native setup

[derekhouck]

Summary

• Adds some clarification to the README for the simple_forms_api module about the setup needed to run the rails command locally as well was the path needed by the command to find the appropriate PDF file.

Testing done

• Ran the rails command referenced in the README successfully using the new instructions.

Acceptance criteria

• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No error nor warning in the console.
• Events are being sent to the appropriate logging solution
• Documentation has been updated (link to documentation)
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
• Feature/bug has a monitor built into Datadog (if applicable)
• If app impacted requires authentication, did you login to a local build and verify all authenticated routes work as expected
• I added a screenshot of the developed feature

Requested Feedback

It would be nice to be able to run this command using the Docker setup, but that would be a more involved fix.

[github-actions]

Backend-review-group approval confirmed.