finatra/http: README.md - add REST best practices
finatra/http: README.md - add REST best practices

Add a "Best Practices" section under "Controllers and Routing".
Also improved the ExceptionMappers section.

RB_ID=678678

joecwu/finatra,andrerigon/finatra,twitter/finatra,joecwu/finatra,yonglehou/finatra,nkhuyu/finatra,tom-chan/finatra,andrerigon/finatra,deanh/finatra,tom-chan/finatra,nkhuyu/finatra,jaume-pinyol/finatra,jaume-pinyol/finatra,jaume-pinyol/finatra,syamantm/finatra,twitter/finatra,tempbottle/finatra,tempbottle/finatra,syamantm/finatra,tempbottle/finatra,deanh/finatra,deanh/finatra,yonglehou/finatra,tempbottle/finatra,syamantm/finatra,jaume-pinyol/finatra,nkhuyu/finatra,yonglehou/finatra,yonglehou/finatra,andrerigon/finatra,andrerigon/finatra,joecwu/finatra,twitter/finatra,tom-chan/finatra,tom-chan/finatra,syamantm/finatra,deanh/finatra,joecwu/finatra,nkhuyu/finatra