Add cache interface and describe use in tests in README

[scevallos]

Raised a Discuss issue for this topic, and providing the kind of interface I'm looking for to use in tests.

---

This change is 

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}