docs: Add CircleCI status badge (#1595)
docs: Add CircleCI status badge (#1595)

This patch adds a CircleCI status badge to our README. This enables users to see how stable the `develop` branch is and allows us to quickly navigate to the `axe-core` landing page in CircleCI (which is something I do frequently).
dequelabs/axe-core,dequelabs/axe-core,dequelabs/axe-core,dequelabs/axe-core