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 there is a sentence in the install section of README:
Note: The rest of this document assumes that you have followed all the instructions in INSTALL.md and updated your CLASSPATH and PATH environment variables according to the Prerequisites section in that file.

So we can assume users have already read the install file when they reach the later part