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.
I think user can use -cp "....:$CLASSPATH" (or %CLASSPATH% on windows), so their system classpath are also preserved.  Maybe you can mention something like that here.