Update Contributor's Guide for Adding Use Cases

[georgemccabe]

Updated instructions for adding a new use case to reflect new changes to GitHub Actions structure and added information that was previously missing

Pull Request Testing

• Describe testing already performed for these changes:
  
  None, really. I just reviewed the changes to the process and added content to reflect the changes.

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  
  Review the new content to ensure it is clear and correct

• Do these changes include sufficient documentation and testing updates? [Yes]

• Will this PR result in changes to the test suite? [No]
  
  If yes, describe the new output and/or changes to the existing output:
  

Pull Request Checklist

See the METplus Workflow for details.

• Complete the PR definition above.
• Ensure the PR title matches the feature or bugfix branch name.
• Define the PR metadata, as permissions allow.
  Select: Reviewer(s), Project(s), and Milestone
• After submitting the PR, select Linked Issues with the original issue number.
• After the PR is approved, merge your changes. If permissions do not allow this, request that the reviewer do the merge.
• Close the linked issue and delete your feature or bugfix branch from GitHub.

[jprestop]

These changes look great! They all seem clear to me, however, I am only familiar with some of these instructions. It's always a work in progress, and we can always continue to make improvements as others follow these instructions. I approve this pull request.