New README.md

@seriousamlqz, @yzhang90, @grosu Please review

@grosu, please assign someone who knows better than me to write the Static Weaving Section: I don't use that option.
1.Briefly introduce what is "Java Agent"
2. "than the second option" is confusing because people read it sequentially. You can say "the easiest option to use" or something like that.