rewrite README.md
Algorithm modification and bug fixed.
A. Some modifications about algorithm are done:
	Initial: only a "diamond" or two subsquent nodes satisfying special conditions can be abstracted according to abstraction method 1. And to ensure the utility of abstracion method 1 under all cases pre-operation method 1 is created. However, now some disadvantages of that have been disclosed: it may bring muddledness after abstraction, and the results are not that beautiful.
	Modified: The object of abstraction method 1 is expanded from "diamond"(two control-flow branches diverge and then merge) to multi-branches diverge and then merge. Hence pre-operation method 1 are no longer useful. I will update the latex tutorial detailedly soon.
B. Some fatal bugs are fixed after testing more complicated CFGs.
Update README.md
Merge remote branch 'ucsb/master'

Conflicts:
	README.md