Add package doc for isolatedfake.
Add package doc for isolatedfake.

R=iannucci@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1359373002

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go