Update OMD Setup Guide for OMD 1.2x

I went ahead and updated the Readme to reflect the changes to the new version of OMD. I also included an example in my OMD setup of how we hide large amounts of performance data from pnp4nagios. Probably should put it all in a script, but it will give people a good idea of how to do that. I'm happy to edit this if you'd prefer to have a shorter OMD section.