HOWTO generate mix-in random trigger files

As described in other HOWTOs, we simulate the effect of detector noise and EM backgrounds in simulation by "mixing-in" or overlaying events collected by a non-physics trigger, usually a 100Hz "random" pulser that runs parallel but orthogonal to the main physics trigger. Generally we are using the `randomtrigger_skim` plugin to generate these mix-in background files. This plugin selects events collected by the random trigger described above, applies a fiducial cut on the electron beam to exclude triggers collected when the beam is off (which would not correctly represent the background), and writes the hit-level data for these events out in HDDM format.

To disable the beam fiducial cut, use the command-line option RANDSKIM:USEBEAM=0.

Detailed scripts and instructions on how to run this process in detail will be provided at a later date.