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.
"should be used" or "can be used" ? I guess there are two questions here:
1. What will happen when -merge is used but no -n aspectName? Is it legal?
2. What will happen when -n aspectName is used but no -merge?