Fixed missing installation and incorrect job instructions in readme

## Problem
According to the code in resque/job.rb, payload classes seem to need a perform method, not a work method. In addition, it might be nice to mention, in order to use resque, one needs to run the rails generator to install first. That way, some slave programmer in the future won't have to suffer the following unhelpfully message when trying to queue up a job.

> NoMethodError: undefined method `[]' for nil:NilClass

## Solution
Changed the part about jobs needing a work method when they actually need a perform method. Add a bit of install instructions about running rails generate resque:install when using with rails."