Add README.md to OSCAL CLI instructions

[DimitriZhurkin]

Committer Notes

Initial draft of the README.md document for using OSCAL CLI. This document is intended for FedRAMP OSCAL end-users (implementers and practitioners).

All Submissions:

• Have you selected the correct base branch per Contributing guidance?
• Have you set "Allow edits and access to secrets by maintainers"?
• 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 added an explanation of what your changes do and why you'd like us to include them?
• If applicable, have all FedRAMP Documents Related to OSCAL Adoption affected by the changes in this issue have been updated.?
• If applicable, does this PR reference the issue it addresses and explain how it addresses the issue?

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

[aj-stein-gsa]

There are some typos and missing info (that I missed in previous review, apologies).