message: adding badge for nightly build
diff: ![CI Build](https://github.com/bark-simulator/bark/workflows/CI/badge.svg)
  ![NIGHTLY Build](https://github.com/bark-simulator/bark/workflows/NIGHTLY/badge.svg)
  +![NIGHTLY LTL Build](https://github.com/bark-simulator/bark/workflows/NIGHTLY_LTL/badge.svg)
  # BARK - a tool for **B**ehavior benchm**ARK**ing
  