fix README: add parse:true documentation

I found error when I followed README to add tags as
    
    @user.tag_list.add("awesomer, slicker") 

The result is one tag named "awesome, slicker". I browsed source and found the right usage should be

    @user.tag_list.add("awesomer, slicker", parse: true)
    # or
    @user.tag_list.add("awesomer", "slicker")

This pull request fixed the missing part in README.