docs(cli): added --config flag

[gabemeola]

Referencing this PR: https://github.com/marionebl/commitlint/pull/261 , I updated the docs to include --config / -g flag for passing a custom configuration.
http://marionebl.github.io/commitlint/#/reference-cli

Description

Added --config, -g to docs.

Motivation and Context

Greater visibility for cli options

Usage examples

// path/to/my/custom/config/commitlint.config.js 
module.exports = {
  extends: ['@commitlint/config-conventional'],
    // Disabling scope-case rule
    rules: {
      'scope-case': [0],
  }
};

echo "your commit message here" | commitlint --config path/to/my/custom/config/commitlint.config.js # pass

How Has This Been Tested?

https://github.com/marionebl/commitlint/pull/261

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[marionebl]

Thanks 👍