Document where to find OSCAL-specific custom metaschema functions not in metaschema java

[aj-stein-gsa]

Committer Notes

Closes #40.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you squashed any non-relevant commits and commit messages? [instructions]
• Have you set "Allow edits and access to secrets by maintainers"?

By submitting a pull request, you are agreeing to provide this contribution under the CC0 1.0 Universal public domain dedication.

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you updated all website and readme documentation affected by the changes you made?
• Have you written new tests for your core changes, as applicable?
• Have you included examples of how to use your new feature(s)?

[RS-Credentive]

Okay, this clarifies my confusion regarding the definition of "has-oscal-namespace" and profile resolution. These are OSCAL specific metaschema extensions. As an author of metaschema tooling, I would like to see a supported mechanism for defining custom metapath functions that can be automatically processed. I will open a separate issue.