Add documentation for CLI commands

[ernilambar]

Fixes #38

[ernilambar]

I developed simple helper CLI command to generate doc for the CLI commands. https://github.com/ernilambar/pc-cli-docs So doc generation is now semi-automatic instead of manual.
It would be great if we could find a way incorporate this command in the trunk and exclude it when we deploy to SVN in the plugin directory.

Preview: https://github.com/ernilambar/plugin-check/blob/38-add-cli-documentation/docs/CLI.md

There is no EXAMPLES section for wp plugin list-checks because it is missing in the command itself. I have created separate PR to add example. #396

[swissspidy]

It would be better to contribute to https://github.com/wp-cli/scaffold-package-command to make it easier to generate documentation for commands within a plugin. Kudos for building something custom. However, long term it is not viable to fork and maintain a custom tool that just works with Plugin Check. Especially since these commands don't change frequently.

[mukeshpanchal27]

Thanks for the PR! I've left some minor feedback for your consideration.