Use relative links for internal content

Relative links should be preferred.

Absolute links are not recommended because
- they're so long which makes them harder to read
- will have to update all the links if
  - the repo is forked
  - the owner of the repo changes
  - the owner's username changes
  - the repo is renamed

For more info, see https://help.github.com/articles/relative-links-in-readmes

P.S. A section on relative links might be useful.