The manual is here:
http://www.vbulletin.com/docs/html/ (remember to choose your version from the drop-down because it defaults to the latest version 4 if you don't). There's a section in the manual called "Plugin System".
However, it's pretty simple. From the admin control panel "Plugins & Products" section choose "Add New Plugin". Choose the hook location (global_start) and give it a name that makes sense to you. Then just enter the code in the box. Don't worry about the other fields.
If you want to read the manual and get more fancy you can create a product, add the plugin to your product, then you can export and import it as you like (for instance, export it so that in case you ever need to do a clean install you can just import it again).
As far as the code, the path name needs to be the actual path on your server. If you're uploading your text file to where everything else is, then you could probably use this:
$mytext = file_get_contents(CWD . "/info.txt");
(ETA: I added a '/' to the above - kh)
Of course you can choose any variable name or file name you want instead of "$mytext" or "info.txt".