Document supported/tested environments

[tbekolay]

NengoOpenCL was mostly intended as a way to run Nengo models on GPUs, but as has been reported in #151 and other places, running NengoOpenCL with CPUs can also give performance improvements. We should start cataloging the hardware devices (and OS / software configurations) that we've successfully run NengoOpenCL on and their performance on some rough benchmark models.