Document

@xiaohe27 @yzhang90 and @kheradmand Please review some document changes
@xiaohe27 Oh Sorry since in README and INSTALL we refer to the pre-build release as zip archive,  I thought you meant that.  
It would be better if we change the sentence to something like this: "Alternatively, you may directly download source code without using git, from here, and unpack it to your favorite location."