message: Add contribution covenant badge
  See
diff: @@ -93,6 +93,7 @@ for general questions and discussion, and please direct specific questions to
  The TensorFlow project strives to abide by generally accepted best practices in open-source software development:
  [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/1486/badge)](https://bestpractices.coreinfrastructure.org/projects/1486)
  +[![Contributor Covenant](https://img.shields.io/badge/Contributor%20Covenant-v1.4%20adopted-ff69b4.svg)](code-of-conduct.md)
  ## Continuous build status
  