Updated README.md
Updated README.md

Added project intro, system requirement, quickstart guide and other
misc info that described this project.

RafuCater/crosswalk-ios,mattWillis/crosswalk-ios,RafuCater/crosswalk-ios,negherbon/crosswalk-ios,hgl888/crosswalk-ios,hgl888/crosswalk-ios,crosswalk-project/crosswalk-ios,ericzhou2008/crosswalk-ios,RafuCater/crosswalk-ios,minggangw/crosswalk-ios,negherbon/crosswalk-ios,ericzhou2008/crosswalk-ios,negherbon/crosswalk-ios,crosswalk-project/crosswalk-ios,negherbon/crosswalk-ios,hgl888/crosswalk-ios,jondong/crosswalk-ios,hgl888/crosswalk-ios,mattWillis/crosswalk-ios,minggangw/crosswalk-ios,ericzhou2008/crosswalk-ios,RafuCater/crosswalk-ios,jondong/crosswalk-ios,ericzhou2008/crosswalk-ios,minggangw/crosswalk-ios,mattWillis/crosswalk-ios,jondong/crosswalk-ios,mattWillis/crosswalk-ios,jondong/crosswalk-ios,crosswalk-project/crosswalk-ios,minggangw/crosswalk-ios,crosswalk-project/crosswalk-ios