Update README now that we don't need anything outside GardeningServer

Now that we don't use qunit, the http server can be started from
within the GardeningServer directory.

TBR=michaelpg@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/489733002

git-svn-id: svn://svn.chromium.org/blink/trunk@180631 bbb929c8-8fbe-4397-9dbb-9b2b20218538