README: Turn the miniKanren.org reference into a link

Some people like me use GitHub stars as a way to bookmark things, and
getting from GitHub to miniKanren.org without an explicit link
involves selecting a piece of text, and copy & pasting it. Making the
reference into a link, I can get there in one click.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Merge pull request #3 from algernon/f/site-link

README: Turn the miniKanren.org reference into a link