Add instructions in documentation to change run.display=-1 in ~/.processing/preferences.txt

By default, Processing launches the sketch window on display 0. In order to support MPE configurations that have multiple graphics cards per host (e.g. Stallion). 

To do this, the user must change the run.display variable in ~/.processing/preferences.txt:

change
run.display=0
to
run.display=-1

We need to add this to the documentation until we add code to modify this automatically.
