Adding note about --drb option for rspec

[BMorearty]

Please pull this request. It's a helpful doc change. It helped me figure out why spork-rails was not working in my app.

[jazminschroeder]

Very useful info in this pull request, If you need help merging this stuff I'd be happy to help.

[arthurnn]

:+1:

First time using spork I was really confused because didnt realize that I had to use the --drb option.

[sahilm]

Thanks.