Add README with guidance

[vanbroup]

This pull request includes updates to the README.md to provide an overview of the repository's purpose, structure, and version control process for the Entrust ECS CP/CPS documents.

• Added an introduction to the repository, explaining its purpose and the tools used for document management.
  • Detailed the directory structure to help users navigate the repository.
• Outlined the CP/CPS version control process, including branching strategy and process steps for managing updates.
  • Included a mermaid diagram to visually represent the Git flow for version control.

[vanbroup]

@bcmorton are the instructions clear, are you missing something or do you see any items that can be clarified further?