Update GBM CLI docs

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Adds a base-level documentation structure for the new release CLI tool.

• Adds base README to CLI root-level directory, based off the README file from the gbm-cli branch.
• Adds README to directory-level commands

In this PR I've added the documentation for the code that is currently merged to trunk. As we continue to add functions and commands to the CLI project, we could continue adding documentation in this format, with a focus on using a README.md file within each directory.