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

This patch replaces the outdated link to the REST API in the docs
directory with the new hosted doc. This would be replaced by the
coming ASCII or Markdown doc in midonet-docs, which would be public
shortly.

Change-Id: I83bdabe30022ef04c5b541cb6817925629f05e80
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>