Adding documentation for running the RRFS forecast ensemble.

[christinaholtNOAA]

Synopsis

Adds documentation to main so users will see it when they come to the repo. The RRFS ensemble support will live in a feature branch, rrfs_ensemble, to point to the appropriate repository and version of the MPAS model to track with RRFS.

Type

• Bug fix (corrects a known issue)
• Code maintenance (refactoring, etc. without behavior change)
• Documentation
• Enhancement (adds new functionality)
• Tooling (CI, code-quality, packaging, revision-control, etc.)

Impact

• This is a breaking change (changes existing functionality)
• This is a non-breaking change (existing functionality continues to work as expected)

Checklist

• I have added myself and any co-authors to the PR's Assignees list.
• I have reviewed the documentation and have made any updates necessitated by this change.

[maddenp-cu]

Forgive my nitpicky comments about YAML configs.