doc: Add `brew install appium` option #13363
Describe the big picture of your changes here to communicate to the maintainers why we should accept this pull request. If it fixes a bug or resolves a feature request, be sure to link to that issue.
Types of changes
What types of changes does your code introduce to Appium?
Relates to Homebrew/homebrew-core#44607
Good to have brew to help with the appium installation. :)