Document how to evaluate the package that will enable ISR behavior through a cache handler

[stevector]

We are currently working on the infrastructure changes and the application level code necessary to accommodate Incremental Static Regeneration behavior.

The infrastructure change is the provisioning of a shared, persistent cache accessible by each runtime container for a given environment (containers can scale horizontally). Separately, Pantheon will need to provide a pattern/package to be implemented at level of a cache handler for a given site to use the cache infrastructure.

I expect that we will first make the infrastructure change available to some or all sites and then work with an expanding set of internal and customer sites to verify that ISR behavior performs correctly. Doing so will require documenting how to implement that cache handler (with appropriate call outs for the fact that the guidance may change) and how to provide feedback or bug reports if the cache handler does not behave correctly.