Include a description of how project vocabulary works from INSTALL.txt

Getting the vocabulary set up properly is the key to getting the project module to function as expected. We need to make it more obvious that this is important and give users more instruction on how to set it up properly. Therefore, we should add a paragraph in INSTALL.txt pointing out how to do this. We also should point out that these instructions exist from README.txt.

See http://drupal.org/node/192982 for original idea.