update readme to remove some duplicated warnings

As suggested by @Cansu and @kheradmand , too many similar warnings may have opposite effects: users may become quite familiar with the warnings so that they will not treat the warnings seriously when they feel they can guess the contents of the warnings. Now I only preserve the **Note** in the 'Install' section. @kheradmand @seriousamlqz please review.
OK, I agree with that. In the very beginning I said we are not teaching Java 101.  Cansu made a mistake by using -cp ., but this doesn't mean we should change the entire thing.