Document LICENSE_FINDER_CLI_OPTS variable

What does this MR do?

This adds documentation for the LICENSE_FINDER_CLI_OPTS variable added here. This variable can be used to specify additional options to the license finder command line tool that is used in the license scan.

An example of how this can be used can be found here.

The gitaly project has both golang and ruby code stored in different directories. In the example provided in the link above the .gitlab-ci-yml template was updated to specify which project directories to scan.

license_management:
  image:
    name: registry.gitlab.com/gitlab-org/security-products/license-management/tmp:05ecf50afea4315e7052fdde061e1c10bc5b17ae
  before_script:
    - sudo apt-get update
    - sudo apt-get install -y libicu-dev libgit2-dev cmake
  variables:
    LICENSE_FINDER_CLI_OPTS: '--debug --aggregate-paths=. ruby'

#13748 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
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• Style guides
• Database guides
• Separation of EE specific content

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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