README.md: Link to the latest manual

Dependencies

• [2] 2IJM73DA Fix link syntax in README.md (#511)
• [3] LRU7K5MW Update build images to be more friendly to users with color blindness.
• [4] E63Q3L3J Build from source remark in README.md
• [5] 2K7ZAIY5 README.md: Point to the manual
• [6] 5LVTBYXA README: fix manual links, fixes #401
• [*] NVMDS7GE Add README.md

Change contents

• replacement in README.md at line 3
  [3.1]→[3.1:176](∅→∅)
  − [Hydra](https://nixos.org/hydra/) is a continuous integration system based

  − on the Nix package manager. For more information, see the

  − [manual](http://nixos.org/hydra/manual/).
  [3.1]

  [3.62]
  + Hydra is a continuous integration system based on the Nix package

  + manager. For more information, see the

  + [manual](https://hydra.nixos.org/job/hydra/master/manual/latest/download-by-type/doc/manual).
• replacement in README.md at line 8
  [2.20]→[2.20:89](∅→∅)
  − [hacking instructions](http://nixos.org/hydra/manual/#chap-hacking).
  [2.20]

  [3.0]
  + [hacking instructions](https://hydra.nixos.org/job/hydra/master/manual/latest/download-by-type/doc/manual#chap-hacking).