Update isolateserver's README.md.
Update isolateserver's README.md.

It was referring to stale documents.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.appspot.com/14509044

luci/luci-py,pombreda/swarming,madecoste/swarming,luci/luci-py,pombreda/swarming,madecoste/swarming,pombreda/swarming,luci/luci-py,madecoste/swarming,madecoste/swarming,luci/luci-py,pombreda/swarming