Add support for JDK's CompletableFuture into the API as part of the Reactor.io API, add docs

[astubbs]

Add API support into reactor.io module for JDK's standard CompletableFuture using same technique as distributed rate limiting example's integration with BucketJ in this diff.

• [ ] Add as convenience method to API in order to more obviously demonstrate the capability.
• [ ] Add docs