add in greylisting info

[mistrm82]

From Rory:
"greylist regions are regions where the input exceeds a threshold, where peak-callers sometimes call spurious peaks. threshold is calculated by calculating depth over the input, sampling it repeatedly and estimating negative binomial parameters and then taking the threshold as the .99 quantile of the NB"

https://github.com/roryk/chipseq-greylist i just copied what the chipseqgreylist R package does