On the home page of a project (tab “Summary”), one can add a project description.
If you want to add a link to local file as well (for instance a PDF to download), use this code:
<a href="http://answers.dataiku.com/local/static/foo.pdf" target='_blank'>test</a>
See http://learn.dataiku.com/howto/code/webapps/static-files-in-webapp.html for where to put foo.pdf on the server.
Updated link can be found here: https://academy.dataiku.com/latest/library/static-files-in-webapp.html