Update dropwizard-client docs (#4289)
Update dropwizard-client docs (#4289)

- Explicitly state to include dependency into project dropwizard-client
- The Rx Usage was out of date. Added RxJava2 example instead.

###### Problem:
1) Problem was that the documentation doesn't explicitly state that it is necessary to add additional dependency `dropwizard-client`
2) Problem was in outdated RxJava usage example that didn't work. 

###### Solution:
1) Added dependency inclusion fragment at the top
2) Fixed Rx Usage example by introducing RxJava2 example

dropwizard/dropwizard,dropwizard/dropwizard,phambryan/dropwizard,dropwizard/dropwizard,phambryan/dropwizard,phambryan/dropwizard