README.md: Use standard markdown syntax

Trivial patch to have the Information page on https://index.docker.io/ look pretty.

Unfortunately right now index.docker.io does not understand extended Github markdown syntax.

Test case: compare
https://index.docker.io/u/aespinosa/jenkins/
with
https://index.docker.io/u/gmacario/docker-jenkins/

Signed-off-by: Gianpaolo Macario <gmacario@gmail.com>