Add addition of zip file to README.txt

Drupal can now be downloaded as both a tar.gz and zip file. The explanation of Drupal core in README.txt describes it as "what you get when you download and unzip a drupal-x.y.tar.gz file from http://drupal.org/project/drupal". This is no longer correct, as you can get Drupal core from a zip file as well now. The attached patch adds this, and also changes the word unzip to extract, as you don't unzip a tar.gz file.
Also fixed a word wrap issue.