README: fix manual links, fixes #401

Dependencies

• [2] 2K7ZAIY5 README.md: Point to the manual
• [3] E63Q3L3J Build from source remark in README.md
• [4] NVMDS7GE Add README.md

Change contents

• edit in README.md at line 2
  [2.8]

  [3.62]
  +

  + [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/).
• replacement in README.md at line 7
  [3.63]→[2.9:337](∅→∅)
  − 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/tarball/latest/download-by-type/doc/manual). For

  − hacking instructions, see

  − https://hydra.nixos.org/job/hydra/master/tarball/latest/download-by-type/doc/manual#chap-hacking.
  [3.63]
  + For development see [hacking instructions]

  + (http://nixos.org/hydra/manual/#chap-hacking).