Drupal comes with a set of TXT files at root level (things like install instructions, upgrade instructions, etc.). To avoid malicious users to have information on the Drupal version you are using, it is strongly recommended to remove those files.
From the root directory of your installation, remove the following files:
Whether or not these files contains security information (like Drupal version for example), the all contain a CVS header that gives a really precise version information on your Drupal installation.