message: [readme] Add badge to README.md
diff: # Starcoin
  +![Build and Test](https://github.com/starcoinorg/starcoin/workflows/Build%20and%20Test/badge.svg)
  +[![License](https://img.shields.io/badge/license-Apache-green.svg)](LICENSE)
  +[![codecov](https://codecov.io/gh/starcoinorg/starcoin/branch/master/graph/badge.svg)](https://codecov.io/gh/starcoinorg/starcoin)
  +
  A Layered Cryptocurrency and Decentralized Blockchain System.
  ## Build from source
  