message: docs(masthead): add codesandbox link for L1 to documentation
  ### Related Ticket(s)
  No related issue
  ### Description
  This adds the codesandbox link for Masthead L1 to the README.
  ### Changelog
  **Changed**
  Masthead README (web components)
diff: @@ -14,8 +14,14 @@ import { cdnJs, cdnCss } from '../../../globals/internal/storybook-cdn';
  > [CodeSandbox](https://githubbox.com/carbon-design-system/carbon-for-ibm-dotcom/tree/master/packages/web-components/examples/codesandbox/components/masthead)
  > example implementation.
  +### Masthead (L0):
  +
  [![Edit @carbon/ibmdotcom-web-components](https://codesandbox.io/static/img/play-codesandbox.svg)](https://githubbox.com/carbon-design-system/carbon-for-ibm-dotcom/tree/master/packages/web-components/examples/codesandbox/components/masthead)
  +### Masthead (L1):
  +
  +[![Edit @carbon/ibmdotcom-web-components](https://codesandbox.io/static/img/play-codesandbox.svg)](https://githubbox.com/carbon-design-system/carbon-for-ibm-dotcom/tree/master/packages/web-components/examples/codesandbox/components/masthead-l1)
  +
  ## Getting Started
  ##### Note: Masthead uses the Carbon White theme by design. Using other themes will not change the Masthead color scheme.
  