Describe how to dispatch commands/queries and handle events from other contexts #110

Currently the Reference Guide does not describe how to dispatch commands and/or queries from one context to another, or how to handle events from other contexts.
This would require a page in the Axon Server section on how to configure a TargetContextResolver for commands and queries, and how to merge several event streams with the AxonServerEventStore#createStreamableMessageSourceForContext(String) method.