Update README.md

Documents the changes that were made in the pull request:
https://github.com/collectiveidea/delayed_job/pull/168/files

Looking at the code in that PR I figured that the max_attempts needs to be set on the payload/job object on which perform also exists and I tried setting it on the custom job and it worked as expected. This feature is indeed very useful and hopefully this is well-documented and maintained in future releases. Greatly appreciate this gem!

A question - would this work with passing :max_attempts in the options? Didn't seem like it would in the code. Also, the DB table does not have the max_attempts as a column so didn't seem like passing it in options would work. 

Seemed like it only worked with the Delayed::Job.enqueue job usage with max_attempts on job set. Also, it looked like it could work on object.delay.some_method usage with the object having max_attempts on it set (since the DelayProxy is instantiated with the object as the payload_class). However, I only tried the first usage, not the second so, let me know if I am wrong. Would be glad to try out the second usage too in case it is needed.