Document CircuitBreaker

[benmccann]

CircuitBreaker appears to be a core concept of Lagom. I'm not familiar with the term in this context, so some brief description would be helpful.

The Javadoc doesn't say much right now: http://www.lagomframework.com/documentation/1.2.x/java/api/com/lightbend/lagom/javadsl/api/CircuitBreaker.html

[TimMoore]

It's a good idea to improve the javadocs, but the concept and implementation in Lagom is also described in the reference guide at http://www.lagomframework.com/documentation/1.2.x/java/ServiceClients.html#circuit-breakers

[benmccann]

Thanks! For some reason that page doesn't show up on the first page of Google when I search for lagom circuit breaker. The only reason that I can guess for that is that the javadocs are hosted on the same domain and rank higher and Google doesn't want to show too many results from the same domain.

I sent a PR to close this out: #385