message: Clarify 'source-root' description to include default path
  This addresses suggestions in PR
diff: ## [UNRELEASED]
  -- The `init` step of the Action now supports a `source-root` input to be able to specify a path under the GitHub workspace to be the root source-code directory. [#607](https://github.com/github/codeql-action/pull/607)
  +- The `init` step of the Action now supports a `source-root` input as a path to the root source-code directory. By default, the path is relative to $GITHUB_WORKSPACE. [#607](https://github.com/github/codeql-action/pull/607)
  - The `analyze` step of the Action now supports a `skip-queries` option to merely build the CodeQL database without analyzing. This functionality is not present in the runner. Additionally, the step will no longer fail if it encounters a finalized database, and will instead continue with query execution. [#602](https://github.com/github/codeql-action/pull/602)
  - Update the warning message when the baseline lines of code count is unavailable. [#608](https://github.com/github/codeql-action/pull/608)
  