Outdated docs in README

[michaelmior]

The documentation in the README shows nosummary as a keyword argument for plot_hist, but it seems as though this was changed to showSummary (and the meaning of the flag inverted). Not sure if there are other inconsistencies.

[kennethgoodman]

Also xlab was added to plot_hist, so "sideways numbers for x-axis of histograms" I think can be taken out as well