Remove the mention of a custom cache from README.md if it's no longer supported

[coolov]

What / Why

Describe how to use the custom cache API if it's still supported. Or if it's no longer supported, remove the mentions of it from README.md.

When / Where / How

Current Behavior

• There is a section in the README.md that shows the usage of Redis as a custom cache, but it doesn't show how to pass the custom cache implementation to fetch:
  https://github.com/npm/make-fetch-happen#example-3
• There is a PR that indicates that the custom cache interface is no longer supported:
  feat(BREAKING): refactor cache handling to be more consistent #49

Expected Behavior

A README.md that is consistent with the supported API.

[kevinlul]

Why was it hardcoded to cacache? This was very confusing navigating this and what I assume is the old @zkat repository.