document plugins dev Gemfile requirements with new core gems structure

related to #4123

when doing plugin development, the plugin `Gemfile` can/should include both logstash-core and logstash-core-event dependencies when needing to point to local source tree for both.

``` ruby
gem "logstash-core", :path => "/some/path/logstash/logstash-core"
gem "logstash-core-event", :path => "/some/path/logstash/logstash-core-event"
```

Note that this is not required but if you need to test your plugin against local sources for either logstash-core and/or logstash-core-event, you need to add them individually.