Add CI badge to README.md

Dependencies

• [2] KH55FNTF Update README
• [3] 2K7ZAIY5 README.md: Point to the manual
• [4] 5LVTBYXA README: fix manual links, fixes #401
• [*] NVMDS7GE Add README.md

Change contents

• replacement in README.md at line 3
  [2.1]→[2.1:149](∅→∅)
  − Hydra is a [Continuous Integration](https://en.wikipedia.org/wiki/Continuous_integration) service for [Nix](https://nixos.org/nix) based projects.
  [2.1]

  [2.149]
  + [![CI](https://github.com/NixOS/hydra/workflows/Test/badge.svg)](https://github.com/NixOS/hydra/actions)

  +

  + Hydra is a [Continuous Integration](https://en.wikipedia.org/wiki/Continuous_integration) service for [Nix](https://nixos.org/nix) based projects.