Updated link to the REST API doc in README.md.

This patch replaces the (now outdated) link to the REST API
documentation with a generic one that will always point to
the latest (release) version.

See also: https://review.gerrithub.io/11322

Change-Id: I9d5557b1eb339c73c715bdd2e65b3496dee1d57e
Signed-off-by: Jan Hilberath <jan@midokura.com>
Merge "Updated link to the REST API doc in README.md."