Add gatling scenario feeder docs and generation script

This commit adds the docs detailing how to modify a gatling scenario
generated by the proxy-recorder into a scenario that has "feeder"
functionality. It also adds a simple generation script that generates
the csv file required by the modified feeder gatling scenario.
Merge pull request #88 from briancain/maint/master/add-feeder-docs

Thanks for adding this!   Looks great.  Add gatling scenario feeder docs and generation script