Update contributing guide for new validations tooling

[aj-stein-gsa]

This is a ...

improvement - something could be better

This relates to ...

• the FedRAMP OSCAL Registry
• the FedRAMP OSCAL baselines
• the Guide to OSCAL-based FedRAMP Content
• the Guide to OSCAL-based FedRAMP System Security Plans (SSP)
• the Guide to OSCAL-based FedRAMP Security Assessment Plans (SAP)
• the Guide to OSCAL-based FedRAMP Security Assessment Results (SAR)
• the Guide to OSCAL-based FedRAMP Plan of Action and Milestones (POA&M)
• the FedRAMP SSP OSCAL Template (JSON or XML Format)
• the FedRAMP SAP OSCAL Template (JSON or XML Format)
• the FedRAMP SAR OSCAL Template (JSON or XML Format)
• the FedRAMP POA&M OSCAL Template (JSON or XML Format)

User Story

As a developer or engineer using, extending, or integrating FedRAMP automation tools into my own work, I would like improved documents about contributing to the newer Metaschema-constraints so I can modify or add them as part of my development work.

Goals

• Summarize the structure of constraint automation components and how they interact
• Explain how to add and modify tests to this repo
• Explain how to add or modify tests in a different code base and integrate them these core tests with contributing upstream

Dependencies

• Updated README in Update validation README to address unresolved review issues #642 is closed to cross-reference with links

Acceptance Criteria

• All FedRAMP Documents Related to OSCAL Adoption (https://github.com/GSA/fedramp-automation) affected by the changes in this issue have been updated.
• A Pull Request (PR) is submitted that fully addresses the goals of this User Story. This issue is referenced in the PR, and it explains:
  • structure of the automation components and interactions of constraints and tests are clear;
  • how to modify a test to contribute to this upstream repo;
  • how to add a new test for new functionality to contribute to this upstream repo; and
  • how to extend tests in a different code base in your own environment to not submit to this repo upstream.

Other information

No response