message: Add status badge to readme
diff: # Voxblox
  +[![Build Test](https://github.com/ethz-asl/voxblox/actions/workflows/build_test.yml/badge.svg)](https://github.com/ethz-asl/voxblox/actions/workflows/build_test.yml)
  +
  ![voxblox_small](https://cloud.githubusercontent.com/assets/5616392/15180357/536a8776-1781-11e6-8c1d-f2dfa34b1408.gif)
  Voxblox is a volumetric mapping library based mainly on Truncated Signed Distance Fields (TSDFs). It varies from other SDF libraries in the following ways:
  