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.
@seriousamlqz  I think when using "-cp ...:$CLASSPATH", the classpath still gets overridden, the difference is that the new value contains the previous CLASSPATH. I'm not sure whether we should assume the users have very weak background in java, so they do not know how to change classpath properly, even if they have been reminded that using -cp will change their default CLASSPATH. I can of course add that tip to the 'Note', but if we assume the users' java background is quite weak, then we have to re-evaluate the document again to think about what other silly mistakes can be made by the users.