Add documentation about backend integration about skylab

[mattt]

SkyLab functionality is limited to probabilistically assigning users to particular conditions, and remembering those results. Any conversions or activities that result from that can be collected any number of ways based on this information, whether by making an HTTP request to your own server, or pinging a logging service using an SDK.

It really just depends.

For lack of any particularly compelling or recommended A/B testing services to integrate with (or at least any that manage to not get acquired within a few months), there hasn't been much pull to develop this library beyond what it already is.