docs(cqrs): Added configurable event bus example

[bradsheppard]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Docs
• Other... Please describe:

What is the current behavior?

Documentation lacks examples on how to use IEventPublisher and IMessageSource as implemented in nestjs/cqrs#12.

Issue Number: #1239

What is the new behavior?

Update documentation which provides an example on how to implement IEventPublisher and IMessageSource.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[darvesh]

Can this be merged? :)

[bradsheppard]

@darvesh Hopefully soon :P

@kamilmysliwiec Do you think its worth adding this? If not I can close the PR.