Fix link syntax in README.md (#511)

Dependencies

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

Change contents

• replacement in README.md at line 7
  [3.63]→[3.177:267](∅→∅)
  − For development see [hacking instructions]

  − (http://nixos.org/hydra/manual/#chap-hacking).
  [3.63]

  [2.0]
  + For development see

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